I actually agree with the majority of what Twilight said above; however, I also feel like adding a bit to some of his points, mostly in the defense of certain pokemon. Because every argument needs both sides heard. Note this isn't me saying 'these pokemon are alright'. It's more me saying how I've dealt with them, or issues I've seen using them myself.
Also I think Hawlucha is bad enough that I'm not even going to play Devil's Advocate and even try and defend it. Hawlucha needs to go.
Obstagoon I'm not actually clear-cut on yet. Firstly; it suffers from 4MMS a bit. Facade takers a turn to turn on; Knock Off only really does something when the enemy has a removable item, and if you're running Obstruct that only leaves a single moveslot. Obstagoon can also be rather difficult to switch in safely. With a x4 weakness to Fighting pretty much any Fighting coverage move on a lure can OHKO it. 90 Base Attack also isn't winning any awards, and Obstagoon is therefor heavily reliant on Guts.
There's also the fact that Obstagoon is really vulnerable to residual damage. Between the damage it is taking from Burn, any other source of residual damage, and any damage it is taking from pokemon either tanky enough to take a hit, hazards, or faster pokemon, Obstagoon usually doesn't last that long.
For example:
252+ Atk Guts Obstagoon Knock Off (97.5 BP) vs. 4 HP / 252+ Def Hippowdon: 130-154 (36.3 - 43%)
252+ Atk Guts Obstagoon Facade (140 BP) vs. 4 HP / 252+ Def Hippowdon: 187-222 (52.2 - 62%) [98% chance to 2HKO after Leftovers recovery]
Keeping in mind this is not the tankiest Hippowdon [252/252+] and this is an Obstagoon which is +Atk and not +Spd. Obstagoon dosen't even do 50% with Knock Off; and the following Facade won't KO without a crit. Hippowdon can just use Slack Off while Sandstorm and Burn whittle down Obstagoon.
Of course; if Obstagoon is running Taunt; then Hippowdon cannot Slack Off on it. However...
252 Atk Hippowdon Earthquake vs. 0 HP / 4 Def Obstagoon: 145-172 (44.3 - 52.5%) -- guaranteed 2HKO after sandstorm damage and burn damage
Taunt Obstagoon is also not getting off freely; especially as it takes at least 3 turns; usually 4; for it to actually KO Hippowdon. Obstagoon actually usually loses to Hippowdon.
There's also quite a few pokemon able to just revenge kill it. Pretty much any common bug-type in the tier [Most notably Ribombee] can do so.
Now; all of these downsides are not to say that Obstagoon isn't too much for UU. However, I do think that Obstagoon is nowhere near the priority of, say, Hawlucha [Which I am of the opinion is completely busted and won't even try to play devil's advocate for]. Suspect Test worthy? Absolutely. I'm not so sure it's quickban worthy however.
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I absolutely think Gengar is likely too much for UU, however, I do feel like I should point out that due to how incredibly frail it is, Gengar is quite easy to lure out and slam on the switch.
Obviously; this takes prediction, but the number of Gengars who try to switch in on a Jellicent they expect to Strength Sap/Will-O-Wisp only to get slammed by a Shadow Ball and OHKO'ed is quite amusing.
I've also had success forceing it out with Drapion, who resists both of Gengar's STAB moves and threatens back a OHKO. With Gengar having trouble getting in in the first place. Gengar also can't run Ground-type coverage for Drapion.
However, when the best argument for something to be staying in UU is 'It's soft so if you predict it you can hammer it' it's not really a very good or convincing argument. If that argument was solid Pheramosa wouldn't be banned in Gen7. I've mostly lost to Gengar as a lategame cleaner, when it's checks are too worn down.
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These two kind of fall into the same boat of disgustingly powerful priority users. Both suffer from immunities to their primary hard-hitting wall-smashing attacks however. They also suffer from being slow and relatively frail. Crawdaunt is also usually Banded, although Diggersby is not; it can be hardwalled by certain pokemon [Driftblim for example is completely immune to both STABs].
I've not had too much issue with Crawdaunt personally, and of the two I'd say Diggersby is more of an issue than Crawdaunt, if only because Crawdaunt is
easier to punish due to usually being banded.
I've seen a lot more Crawdaunt that Diggersby; and it's reliance on Choice; fragility and vulnerability to Hazards usually leads to it only really being able to come in after something faints, and even then, only a few times.
The key difference between the two IMO really is the fact that Crawdaunt is easier to punish than Diggersby. Diggerby's strength comes from Huge Power and it usually isn't Choicelocked. It's also faster and bulkier than Crawdaunt. Crawdaunt can come in, wreck something and then usually be forced out of punished. Diggersby can... switch moves. It's not locked into Quick Attack and can switch to it's harder hitting moves against walls or pokemon who don't threaten a OHKO. Crawdaunt is usually stuck into Aqua Jet/Crabhammer and cannot swap.
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I'll be completely honest, I've not seen enough Haxorus post-Dynamax to really judge it fairly.
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Mamoswine just simply always catches me by surprise. I always think it'll do less damage than it does; or that it'll take more damage than it does. I constantly under-estimate it. At the same time... I don't really see it too much either [despite it's high usage stats... I just keep missing Mamoswine users I guess?] My bad playing around Mamoswine makes it a little difficult for me to say if Mamoswine is good or Too Good.
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I would like to see some discussion on some of these Pokemon or others, as we are now in the stage where stuff can be looked at. I also hope that if the council is planning to Quick Ban a/multiple Pokemon, that they take the opportunity to at least encourage discussion here on said Pokemon first or at the very most their reasonings as to why X was banned. Thanks for reading this long post if you did.
I too would quite like at least a discussion thread in a similar vein to that of Galarian Darmanitan in OU preceding any quickban. Not a full suspect thread but at least one where people can discuss for about a week. Give people a chance to find something, identify the problem set and so on. Even if I don't
quite agree with the end result of the G-Darm thread [I personally think the precedent set by Gen 5's treatment of Drizzle and Politoad should have meant a Gorilla Tactics ban to preserve Zen Mode Darm was the right move; not a G-Darm ban. Also Power Construct in Gen7.]; I think the thread itself was right.
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IMO:
Quickban Worthy: Hawlucha; Gengar
Suspect Worthy: Obstagoon
Sorta 50/50 on: Diggersby
Need to see what happens after the above three are dealt with to really make up my mind; but leaning 'They're just very good': Crawdaunt; Mamoswine
Not enough experience to make an honest statement on: Haxorus