You're implying Dragon coverage is harder than it actually is. You don't need offensive dragons to beat other dragons; more defensive Dragon or Fairy-type mons such as Dialga, Xerneas, and Magearna, or just the rogue Core Enforcer/Moonblast/Ice Beam on a mon otherwise walled by Giratina/Zygarde does just fine pressuring them. Giratina and Zygarde-Complete rarely can switch into MMY, and it's not Dragon-type.
Actually, it would:
252+ SpA Choice Specs Aerilate Charizard-Mega-Y Boomburst vs. 252 HP / 252 SpD Giratina: 364-430 (72.2 - 85.3%) -- guaranteed 2HKO
Charizard-Mega-Y was just never used because Rayquaza-Mega existed.
And another argument: Aerilate Rayquaza doesn't really need Draco Meteor that much to beat Dragon-types, anyway:
252+ SpA Choice Specs Aerilate Rayquaza-Mega Boomburst vs. 252 HP / 252 SpD Giratina: 402-474 (79.7 - 94%) -- guaranteed 2HKO
252+ SpA Choice Specs Aerilate Rayquaza-Mega Boomburst vs. 252 HP / 252 SpD Zygarde-Complete: 472-556 (74.2 - 87.4%) -- guaranteed 2HKO
Draco Meteor was kinda nice for getting the jump on Regenvest (even though it doesn't actually OHKO either of them and Boomburst 2HKOd them anyway).
Considering that only 8.8% of the top 15 mons last October (the most recent month with only Gen 7 BH as an option, in order to have a high sample size) used Unaware, I think it's safe to say Unaware is not a common issue.
This represents an even lower amount of Unaware users. I cannot off the top of my head think of a good Unaware non-defensive self-improofing mon. Except maybe Arceus, but the amount of Arceus using Shell Smash is a bit higher than 1% so again, not a common issue you'd consciously have to think about.
Not sure why resorting to coverage moves is a bad thing. You talk about 4MSS but SF MMYs have almost always packed coverage for Giratina and Zygarde-Complete, and almost none of them have packed coverage for Shedinja (though some use Knock Off or hazards). If they have problems with Shedinja they prepare their other teammates to handle it, such as slapping on the funny volcano man or having a Mold Breaker Pursuit mon in the back.
Why is Photon Geyser somehow "forced" when it's on MMY? It runs it frequently on MG sets, and most Choice Specs sets use it or Moongeist Beam which also hits opposing Psychics and Giratina. And why would MMY need to have Photon Geyser be physical? Its physical coverage hits what it needs to anyway, and it really wants to hit specially defensive mons it can just use Psystrike. Again, if it doesn't hit Shedinja, the team can be modified a little to pressure Shedinja.
Bolt Strike is only used on sets that don't have room for Psystrike (meaning only mixed non-STAB MG and most SF variants). Most MMYs can hit Kyogre.
It decides between Moonblast or Ice Beam because the former hits MMX harder. You imply Clanging Scales does what Moonblast and Ice Beam both do, which is inaccurate because Necrozma-Ultra gets forced out by MMX so it wouldn't be able to hit it in the first place.
This is mostly correct, although you forgot that MMY has about 6x the bulk of Deoxys-Attack (assuming that MMY is Naive and Deoxys-Attack has 0 defensive investment to maximize damage against Imposter), so it can afford to switch in and not worry about 2HKOing the opponent while being OHKOd back. But one can compare both Necrozma-Ultra and Deoxys-Attack to MMY, and discussion about Deoxys-Attack is irrelevant anyway.
In conclusion, Necrozma-Ultra is fine in C rank with other situational substitutes because its competitor, MMY, is much more versatile and at a much better speed tier. Having a mixed but ultimately rigid set allows it to fit with its C-rank companions.
1. That’s a lie. I am not implying that Dragon coverage is hard, what I am implying is that having a Pokemon that already packs Dragon coverage means you are less likely to need it on the other members of your team.
2. Again, not a 1HKO, and how can Charizard-Mega or even Rayquaza-Mega expect to even 2HKO when Core Enforcer removes their STAB, and then turns it into a Normal-type which Giratina is immune to?
Core Enforcer is a 2HKO on Rayquaza-Mega, which compounds with its Stealth Rock weakness... Giratina can recover on Rayquaza-Mega’s forced switch out, while Rayquaza-Mega can not touch Giratina once Core Enforcer hits, afterall, you did use Choice sets...
252 SpA Griseous Orb Giratina Core Enforcer vs. 252 HP / 252 SpD Rayquaza-Mega: 258-306 (62.3 - 73.9%) -- guaranteed 2HKO
I brought up Draco Meteor so that way Soundproof wouldn’t be a factor, as Boomburst didn’t hit Soundproof, but even skipping Soundproof, Giratina surviving Boomburst while Boomburst is effectively negated puts Giratina in as a Check / revenge kill position, and makes Rayquaza susceptible to being KOed itself.
Even without its Griseous Orb, it’s a 2HKO against Rayquaza.
252 SpA Giratina Core Enforcer vs. 252 HP / 252 SpD Rayquaza-Mega: 218-258 (52.6 - 62.3%) -- guaranteed 2HKO
To prevent situations, like the one listed above, Specs Draco was the deciding factor, in being a secure way to handle Zygarde-Complete / Giratina, KOing against non-AV.
3. I never said it was common, I did say it was viable enough to consider when building a team. Plus an almost 10% usage rate isn’t that uncommon, further you only listed the top 15 Pokemon, whom are not always defensive, and thus when I referred to defensive Unaware, like Chansey I wasn’t referring to same set self-Improof mons.
It would be better to look at Defensive Pokemon, since you specifically said “non-Defensive”, and for the record I wasn’t referring to “same set self-Improofing” mons, I was referring to using an Unaware teammate to handle your sweep. Afterall, we all know Chansey doesn’t sweep with Unaware, and me listing that proves I was referring to using Unaware overall. In the end, if you cannot same set self-Improof, you can use a teammate to switch in on Imposter with Unaware, like Chansey, especially since their Imposter won’t pack Life Orb, or a Z-move.
Think of Unaware Yveltal Improofing Shell Smash Dazzling MMX.
4. Resorting to coverage moves isn’t necessarily a bad thing, but relying on coverage moves, especially when you have 4 move slot syndrome is. It prevents you from having enough space to cover more since you don’t have the necessary STAB for strong nuetral hits, nor do you have the STAB to boost a super effective hit, forcing you to run items or abilities that make up for that, which can limit the sets you can run. I.e. SF MMY Ice Beam Vs Moonblast: While both can hit Dragons, Moongeist Beam becomes more necessary to hit MMX if you don’t use Moonblast, but if you used Moonblast you could skip Moongeist Beam for MMX, but then you now need Photon to hit Shedinja. This isn’t a problem when a STAB Photon off of Adaptability can KO Shedinja and threaten MMX with chip damage, or just Z-Move if it needs to secure a KO at full HP.
Thus, by doing what it does without resorting to having to swap in moves and swap out other coverage moves, it can squeeze more and therefore cover more.
I have seen MMY with Photon or Moongeist Beam, and people sometimes debate on Photon or Psychic for STAB due to the increased base power of Psychic Vs Moldy move effect on Discord, and even on Smogon.
5. So I wasn’t referring to it being forced on MMY, I was referring to how on Pokemon without STAB it would be forced since it is a common use to have Moldy moves on sweepers/wall breakers, but it’s not considered a coverage move on Necrozma-Ultra because it would use it anyways. I then acknowledged how Rayquaza-Mega would use Sunsteel Strike; Moongeist Beam on it’s sets and that it often created 4 move slot syndrome.
6. The point is, Necrozma-Ultra doesn’t need Psystrike, or Bolt Strike, it can simply use Photon off of its Attack stat, and it can use Clanging Scales to not rely on Light for Ruin for a MG set. This ultimately allows it to not worry about Fur Coat like Psystrike Mewtwo would need to consider for Psystrike, not worry about Shedinja. What Clanging Scales does is allow Necrozma-Ultra to rely less on picking the right move, since it’s pretty safe to use that without selecting the wrong coverage move with a lot of resistances (Dragon has 1 resistance and 1 Immunity), while Ice Beam and Moonblast are much easier to resist. Also it’s Z-Move KOs MMX, so it doesn’t rely on coverage moves. MMX doesn’t necessarily threaten out Necrozma-Ultra since it’s STAB is resisted, it’s coverage move in V-Create is resisted, and it wouldn’t want to come in and take a huge hit from its neutral moves anyways. Sure it can come in on a slow pivot; and threaten with Spectral or U-Turn, but you are not factoring in Necrozma-Ultra’s fourth move which could be Shell Smash, or another move that can threaten MMX.
7. Deoxys-A was compared to MMY in the sense they run the same sets. If either runs Focus Sash, then it’s kind of irrelevant how much bulk it has, as MMY is unlikely to survive more than 2 unresisted hits anyways. The fact you are saying you can compare MMY to Deoxys-A and Necrozma-Ultra is because they are mixed Psychic attackers, just like Necrozma-Ultra is a mixed Dragon attack (like Rayquaza). Their secondary typing difference makes them different, but their primary typing makes them worthy of the comparison when they can share the same STAB to nab the same KOs.
8. Necrozma-Ultra is above C, because it is more flexible in its Movesets, stats, and dual typing while other Pokemon in C rank like Metagross-Mega or Swampert-Mega are limited in what they can do; while Necrozma-Ultra is fast and powerful enough to perform a variety of sets.
comparing it to good offensive pokemon makes ultra look much more viable than it actually is. one might compare it to mmx, mmy, kyurem-w, ray, and even gar in terms of offensive capabilities. while these comparisons are not technically incorrect, they fail to account for ultra's abysmal disadvantage state.
ultra needs the momentum stacked in its favor in order to come in at all. most pokemon can 2hko it with moves like spectral and core, which is really bad considering ultra's reliance on zmove to ohko the majority of its targets (meaning that being chipped shortens the list of what it can threaten, and using up zmove after that makes it a complete non-threat). in addition, ultra is easily revenge killed by both mmy and gar, 2 very threatening pokemon, so even if it manages to get a kill, the effort needed to both get it in and deal with the momentum shift afterward is often not even worth it at all. this also means that ultra gets absolutely invalidated by uturn.
considering the other 2 sets, smash and specs: smash is very annoying to imposter-proof and isn't horrible as a smasher, but suffers the same issue of being hard to get in. specs has a basically permanent zmove in the form of psycho boost (w psystrike for ogre) meaning that it can get consistent ohkos, but it must be used on a very slow and passive team; as a result, it often simply fails to break faster than the opposing team. being prediction-reliant and vulnerable to midgrounds exacerbates this even more.
these reasons are why i do not support ultra rising above c rank.
What about Soundproof Clanging Scales? If it packs Shell Smash to make up for not using Adaptability, it can Imposterproof without much effort, and just like Soundproof Rayquaza-Mega / Sceptile-Mega had, it can use a Z-Move for the Imposter.
Improofing it via Soundproof Slowbro-Mega is easy, just like it was with Diancie-Mega / Rayquaza-Mega, and it provides a helpful slow pivot to boot.
+2 252+ Atk Adaptability Necrozma-Ultra Photon Geyser 252 HP / 252+ Def Slowbro-Mega: 136-160 (34.5 - 40.6%) -- 54.2% chance to 3HKO after Leftovers recovery
252+ Atk Adaptability Necrozma-Ultra Photon Geyser vs. 252 HP / 252+ Def Slowbro-Mega: 68-81 (17.2 - 20.5%) -- possible 7HKO after Leftovers recovery
In my original set, with the link, I provide calculations that use Life Orb for solid KOs and do not include any boosts from Shell Smash, in fact I don’t even include a Z-move calc.
Life Orb alone allows it to 1HKO the walls, so I don’t know why you said it needs a Z-Move... here is the list of calcs, since you seem to think it cannot use Life Orb to great effect?
252+ Atk Life Orb Necrozma-Ultra V-create vs. 252 HP / 252
+ Def Registeel: 367-432 (100.8 - 118.6%) --
guaranteed OHKO
252 SpA Life Orb Adaptability Necrozma-Ultra Clanging Scales vs. 252 HP / 252 SpD Zygarde-Complete: 619-728 (97.3 - 114.4%) --
81.3% chance to OHKO after Leftovers recovery
without Steath Rocks
252 SpA Life Orb Adaptability Necrozma-Ultra Clanging Scales vs. 252 HP / 252
+ SpD Zygarde-Complete: 562-665 (88.3 - 104.5%) -- 68.8% chance to OHKO after Stealth Rock (25% chance without Steath Rocks)
252+ Atk Life Orb Adaptability Necrozma-Ultra Photon Geyser vs. 252 HP / 252 Def Kyogre-Primal: 320-377 (79.2 - 93.3%) -- 43.8% chance to OHKO after Stealth Rock
252 SpA Life Orb Adaptability Necrozma-Ultra Clanging Scales vs. 252 HP / 252 SpD Giratina: 530-624 (105.1 - 123.8%) --
guaranteed OHKO
Additonal:
252+ Atk Life Orb Necrozma-Ultra V-create vs. 252 HP / 252+ Def Necrozma-Dusk-Mane: 411-486 (103.2 - 122.1%) --
guaranteed OHKO
Where did it need a Z-move to wallbreak?
—————
2HKOed by Core Enforcer?
Rayquaza-Mega is barely bulkier than Necrozma-Ultra
(105 HP / 100 Def / 100 SpD VERSUS 97 HP / 97 Def / 97 SpD)
When does Rayquaza-Mega survive a Core Enforcer at a 3HKO that Necromoza-Ultra would only survive at a 2HKO?
Spectral Thief is a weakness of Necrozma-Ultra, but that’s not as bad as a 4x weakness to Ice, and a Stealth Rock weakness - which upon switch-in more than cancels out the minor bulk advantage.
Spectral Thief is used on how many Pokemon faster than Necrozma-Ultra? MMX, Maybe MMY? We know Beedrill-Mega, Aerodactyl-Mega, and Gengar-Mega wouldn't use it, and oftentimes even Pheromosa prefers Knock Off (which does less if holding a Z-Crystal) to cripple the foe. Sure, they have other moves, but if they are Choice locked into something it resists like V-Create, Secret Sword, or Close Combat, then it’s not like you would be KOed. Also, if they do use Spectral as a faster Pokemon, who is to say you haven’t used Shell Smash on their slow pivot in?
They would need Focus Sash to survive, and since the few faster Pokemon I listed as using Spectral Thief are prettt much limited to MMX, who is less likely to use Focus Sash over Choice Band or Life Orb, it doesn’t seem as likely.
So since only a couple of Faster Pokemon might use it, that means it comes from slower Pokemon, so what is going to tank the first hit to use Spectral Thief to begin with?
Plus if it gets 2HKOed by something slower that can 2HKO it, by virtue of it being faster, it can 2HKO them first so it still forces them out...
——————
Post unchallenged, guess I am right!
Primal Groudon (
Mega Rayquaza (
Garchomp-Mega
Registeel
Aerodactyl-Mega
Arceus
Ho-Oh
Magearna
Necrozma-Dusk-Mane
Aegislash
Blaziken-Mega
Cresselia
Greninja-Ash
Slowbro-Mega
Solgaleo
Zekrom
Charizard-Mega-Y (Poison Heal, Aerilate, Triage)
Deoxys-Attack
Metagross-Mega (Magic Bounce, Regenerator, Tough Claws)
Necrozma-Ultra
Palkia (Drizzle, Tinted Lens, Swift Swim)
Pheromosa
Rayquaza (Aerilate, Triage, Tough Claws)
Sableye-Mega
Scizor-Mega (Flash Fire)
Steelix-Mega (Flash Fire, Prankster, Regenerator)
Suicune (Magic Bounce)
Swampert-Mega
Tapu Fini (Fur Coat)
Aggron-Mega (Regenerator, Prankster)
Ampharos-Mega (Prankster, Galvanize)
Darmanitan-Zen (Regenerator, Fur Coat)
Diancie
Gardevoir-Mega (Tinted Lens, Pixilate)
Giratina-Origin (Magic Bounce, Prankster, Mold Breaker)
Heatran (Magic Bounce, Levitate)
Heracross-Mega (Triage, Tinted Lens)
Hoopa-Unbound
Lucario-Mega (Steelworker)
Lugia (Aerilate, Magic Bounce)
Marowak-Alola (Tinted Lens, Tough Claws)
Nihilego (Sand Stream, Levitate)
Regirock (Sand Stream)
Reshiram (Turboblaze, Poison Heal, Desolate Land)
Sceptile-Mega
Stakataka (Steelworker, Sand Stream, Levitate)
Toxapex
Venusaur-Mega (Flash Fire)
Xurkitree (Galvanize)
Zeraora (Snow Warning, Teravolt) 
Mega Latias
Deoxys-Speed
Mawile
Mega Alakazam
Mega Salamence
Latios (Contrary, weather ability, Mold Breaker, Regenerator)
Landorus-Therian (Prankster)
Pokestar MT (Flash Fire, others)
Archeops (Poison Heal, Magic Guard)
Infernape (Tinted Lens, Magnet Pull)
Jirachi (Dry Skin, etc)
Latias (see Latios)
Mewtwo (Sheer Force, Adaptability, some others)
Shaymin (Poison Heal, Drizzle)
Tyranitar (Sand Stream, Levitate, Dry Skin, Poison Heal)
Arceus-Flying (Multitype)
Arceus-Rock (Multitype)
Arceus-Steel (Multitype)
Darkrai (No Guard, Poison Heal)
Genesect (No Guard)
Keldeo (Tinted Lens, Drizzle, Poison Heal)
Landorus (Sheer Force)
Raikou (Dry Skin)
Scizor (Unaware, Dry Skin)
Shaymin-Sky (Sheer Force)
Terrakion (Magic Guard)
Thundurus-Therian (Sheer Force)
Shuckle (Unaware)
up to d: im not super confident on this nom but 
Agree, there are other breakers that do around the same work while pulling their weight in many other matchups.