I have many reasons to believe that Genesect's and Reshiram's viability in the Gen 8 SS Ubers tier are falsely represented in the viability rankings. I have decided to write them down so here they are:
Genesect
Firstly the scarf set. Choice scarf Genesect is incredible for it's 99 speed tier outspeeding most other scarfers a lot of sweepers at +1 speed and basically everything else. It can also easily be slapped onto any team given its incredible traits and incredible utility. In a pinch it can take at least 1 attack from most pokemon in the tier, Dynamax Cannon from Eternatus, Astral Barrage from Calyrex-Shadow etc. Most commonly you are going to be switching in on defensive Xerneas' Moonblast, NDM or just being lead off.
One of its best traits is being a perfect lead, because whatever your opponent leads off with you are guaranteed initiative and good damage using U-turn (unless their lead is Calyrex). You are going to want to pair it with a strong wallbreaker such as Groudon or Zekrom and other strong attackers like Weavile (who is an especially good partner) to take full advantage. The amount of pokemon it checks and how well it performs in the endgame are its best qualities. It can check so many pokemon that it will be shorter if I just list the pokemon it doesn't have a good match up against and those would be: Eternatus with over 60% Hp, Calyrex-Shadow (only the scarf variant), Ho-oh and Dialga. Those are the only factors stopping Genesect from U-turning over everything forever.
It should be partnered with a pokemon that can switch in decently into Eternatus like NDM and Ho-oh like Kyogre, of course it should also be partnered with a pokemon that can abuse them switching in like a Groudon. Since every team should have a switch in to those two Genesect is easy to fit on and giving something as dangerous as a Groudon free turn can allow for an easy advantage since the only other way you're getting something like Groudon in on Ho-oh is on a double switch or Yveltal's U-turn which is often hard to slot in given how many other incredible utility moves it has access to like Sucker Punch, Taunt or Defog which are all important for it's function. And giving yourself an easy way to lure those pokemon in for an ever greater advantage or simply forcing damage on something like Yveltal or NDM is a very efficient way to quickly make progress.
I have been saying how easy it is to slap on a team and that is because it has incredible utility beside its incredible U-turns.
Firstly, it has a stab Iron Head that can help with whittling down Xerneas and for a strong STAB that can hit a lot of pokemon for neutral damage.
Secondly, how easy it is to pick up the right boost with download. Everything that you're U-turning on is giving you an attack raise and everything you're hitting with Ice Beam is giving you a sp.atk raise. The only exceptions are Xerneas who is being hit with Iron Head anyways, offensive Eternatus with 4 sp.def and Garm with 4 sp.def. Thirdly, priority in Extreme Speed.
The third point is pretty self explanatory, for revenge killing purposes when your speed isn't enough such as when facing a boosted Xerneas or Black Glasses Yveltal on low health or when facing Trick Room teams. Against those you can get in when Hatterene's sash is broken or is simply on low health to get an attack boost, and then hit the pokemon coming in with a boosted Extreme Speed. For these reasons it's a good idea to pair scarf Genesect with other priority such as Sucker Punch Yveltal and especially Shadow Sneak Marshadow to be able to revenge kill even with just priority attacks when Extreme Speed isn't enough or more commonly when the other priority isn't enough.
Lastly, its dominance in the endgame. When it's checks are weakened or already knocked out Genesect really thrives becuase it's great in insuring wins, there won't be a specs or sash Calyrex-Shadow aiming to snowball trough your weakened team (U-turn twice), there won't be a Marshadow about to clean up (Iron Head), there won't be Zekrom or Rayquaza about to Dragon Dance and sweep you (Ice Beam), Xerneas about to boost and sweep (Iron Head if unboosted/Extreme Speed if boosted) etc. All of these late game threats are dominated by Genesect and it has a tool to deal with ALL of them. That's why it's scarf set is so incredible.
Now the choice band set. Choice band plays a lot differently then choice scarf since it trades away it's late game usefulness for and insane power boost. Its U-turns are now jacked up to insane levels of power which are 2hkoing almost everything that would give it an attack boost except of course Eternatus and Ho-oh. Speaking of those two, its Extreme Speed is now its new best attack since it is 2hkoing both of them as well as everything that hasn't got a heavy physical defense investment or resists the attack. Now everything that falls to around half health is in danger of being picked off by Espeed and that is most likely the result of its previous attacks. Adamant nature with Blaze Kick over Ice Beam is an incredible new option since it easily 2hkos NDM which could normally try to heal off its U-turns which were already quite threatening and of course ohkoing Ferrothorn. As such it is more demanding and harder to fit on teams but it's an incredible offensive partner for almost anything that can capitalize on weakening NDM and luring Ho-oh and Eternatus like scarf mix Zekrom with Volt Switch who is especially good because they can create a deadly Volt-Turn combo.
Simply, what really makes Genesect so amazing and consistent is because every team can heavily capitalize from what it brings to a team which would be a back up Xerneas check, strong revenge killer, great neutral lead, momentum and initiative. All it takes for a team to make the most use of it is a bit more offensive play.
I have no idea what you guys are doing by placing Genesect in the D tier.
Reshiram
Reshiram is an incredible wallbreaker because of the ease with which it claims kills once the sun is set up. Ohkoing all of Yveltal, Eternatus, NDM, Xerneas, Marshadow, Calyrex-Shadow, Groudon etc. (I'm not gonna list out the entier tier just take a look yourself). The only pokemon it can't ohko would be a much shorter list to compose. Those would be Kyogre (2hkod by Draco), Blissey (Body Press 2hkos), Tyranitar (Body Press almost ohkos) and Ho-oh (rock coverage with comes close to ohkoing). As you can see the only pokemon that don't die in a single hit are 2hkod anyways. A lot of people love saying "Rampardos Theorem" or other such bs, but that doesn't at all translate to real battles. They obviously didn't give Reshiram a proper try because once it hit's the field the opponent is at the least taking a Draco with Kyogre (And only the defenisve variants check it) and most likely it is taking at least one ko, so the trouble of getting onto the field is well worth it.
As for its teammates, a Groudon is necessary (preferably a bulkier support variant with Thunder Wave) but not with Stealth Rock. DON'T USE ROCKS WITH RESHIRAM!!! Don't waste time on hazards when you should be clicking twave or claiming your ko with Reshiram. What is the point of trading rocks if your Reshiram is taking 25% when coming in and everything on the opponent's team has boots or resists rocks anyway? Effort in preventing your opponent's hazards goes a whole lot longer way than trying to set yours up. The only scenario where hazards are acceptable on a team that Reshiram is on is if you're using a HDB variant. As for different sets, the best would be life orb to not only strengthen its Blue Flares and Dracos but also Body Presses and Rock Slides (I use it over SE because I don't want to miss), although all of HDB, specs and scarf are also viable. It's just that the life orb set does the role of wallbreaking most effectively and consistently. A water resistant pokemon that can help pivot around Kyogre is also of great use like scarf Palkia or scarf Zekrom.
I spent less effort in explaining how Reshiram works because it is more simple, get up sun, get in Reshiram, claim your kill. I think Reshiram should be partnered up with specially defensive Groudon variants to lure opposing grounds that Reshiram demolishes, spread paralysis to transform something like Yveltal or Eternatus from a tool to threaten Reshiram out into a free ko for it as well as being able to switch into Eternatus, take a hit from Kyogre as well as most other pokemon and Thunder Wave them back. If you want to use offensive Groudon and a strong special attacking partner that can threaten opposing grounds I would suggest AV Kyurem-W for it's ability to switch into Kyogre enough times and allow Groudon to claim kills easily. In that scenario I am again suggesting not to use Stealth Rock on the team.
Because of these reasons I think that Genesect and Reshiram posses a place in the metagame. As you can presume if it were up to me Genesect would be S- and Reshiram would be A-, but I will be realistic so I am suggesting A- for Genesect and C+ for Reshiram.
A- (1) --> A (2)
B+ --> A- (2)
B --> B+
B- --> B
B- --> B
B- --> B
C --> C+
D --> C-
UR --> C-
UR --> C-
D --> C-
S (4) --> S- (1)
S- (1) --> S- (2)
S- (2) --> S- (3)
A (2) --> A- (1)
A (4) --> A- (3)
A- (2) --> B+
B+ --> B
B --> B-
B --> B-
B- --> C+
B- --> C+
B- --> C+
C+ --> C
C+ --> C
C+ --> C
C+ --> C
C --> C-
C --> C-
C- --> D
C --> UR
C --> UR
C --> UR
C --> UR
C- --> UR
C- --> UR
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