Kommo-o was a pretty meh Pokemon back in SM. It had decent stats but an odd movepool with its best physical Fighting move being Sky Uppercut. Then USUM came around and brought move tutors, Close Combat and the Kommonium Z. 185 STAB Dragon Move that raises all of your Base Stats? That is insane!
Positives:
+ Clangorous Soulblaze is an amazing Z-Move, being able to raise all of your stats by one stage. This can make Kommo-o very difficult to deal with
+ This thing's movepool is huge. Physical wise, you have Superpower, Close Combat, the Elemental Punches, Iron Head, Aqua Tail, Dragon Claw, Poison Jab, Earthquake etc. For Special, you have Clanging Scales, Focus Blast, Flamethrower, Flash Cannon etc.
+ 110/100 Offenses are pretty solid
+ 75/125/105 Defenses are also pretty good
+ 3 pretty good abilities
+ This thing is very hard to stop when it starts. There are very few Pokemon like that
+ Did I mention Clangorous Soulblaze?
Negatives:
- Unfortunately, this isn't Gen 5 when Fairies didn't exist. If a Fairy switches into your Clangorous Soulblaze, your Z move is wasted and you don't get the boosts. In doubles, only 1 opponent has to be a non-fairy but still...
- Speaking of Fairies, you have a 4x weakness to them. In addition to a weakness to Flying, Psychic, Dragon and Ice
- 85 Speed isn't bad, but it could be better. (I mean you get +1 Speed from using CS but even then, stuff such as T Koko can still outspeed)
- 100 Sp Attack is certainly solid, but again, it could be better for a Sweeper.
- You might not have many friends left if the CS strategy works.
Abilities:
Bulletproof: Pretty good, considering that there are quite a few moves such as Focus Blast that would be annoying to be hit by. But the moves blocked by Soundproof are generally better.
Soundproof: Having an immunnity to Boomburst, Mega Salamence's Hyper Voice or Mega Gardevoir's Hyper Voice or even other Kommo-o's Clangorous Soulblaze. That's pretty cool.
Overcoat: Having an immunity to weather is OK, as well as an immunity to Spore, but Soundproof is still generally better.
Counters: Any of the Guardian Deities can absorb a Clangorous Soulblaze and OHKO back with a STAB Fairy-Type move (except from Tapu Bulu). Tapu Lele does have to watch out for Flash Cannon though. Alolan Ninetales can OHKO with a Moonblast and absorb a CS although again, it has to be careful of a Flamethrower or Flash Cannon. Vanilluxe can do a similar job if it runs a Choice Scarf; assuming Kommo-o hasn't got its boosts yet. Mega Altaria is immune to CS and can OHKO back with a Fairy/Dragon move. However, a Hyper Voice will probably be blocked by Soundproof. Finally, Mega Metagross can take anything but a Flamethrower (and isn't OHKO'd usually) and deal massive damage back with a Zen Headbutt.
It can be pretty difficult to counter Kommo-o when it has its boosts. Preventing it from getting the boosts is a good starts though.