Cofagrigus should beat it pretty consistently in a 1v1 due to Pain Split taking advantage of Raticate's naturally high HP for a full or nearly full heal and then Cofag can proceed to Will-o-Wisp and stall out Gravity. Or it'll just start running Mummy in order to completely mess it up.This looks pretty broken.
It 2HKOs basically everything except Tapu Bulu with its unresisted combination of Knock Off, Double-Edge, and High Horsepower, and that's not even mentioning Choice Band or Swords Dance. Unless some physical behemoth is introduced, this mon looks like the death of any defensive team.+2 252 Atk Raticate-Alola High Horsepower vs. 248 HP / 252+ Def Celesteela: 196-232 (49.3 - 58.4%) -- 65.2% chance to 2HKO after Leftovers recovery
+2 252 Atk Raticate-Alola Knock Off (97.5 BP) vs. 252 HP / 252+ Def Cofagrigus: 100-118 (35.2 - 41.5%) -- guaranteed 3HKO (note that Cofagrigus does not have reliable recovery)
+2 252 Atk Raticate-Alola Double-Edge vs. 248 HP / 252+ Def Tapu Bulu: 172-204 (46.1 - 54.6%) -- 7.4% chance to 2HKO after Grassy Terrain recovery
+2 252 Atk Raticate-Alola High Horsepower vs. 248 HP / 252+ Def Swalot: 262-310 (47.3 - 56%) -- 80.1% chance to 2HKO
But yeah nothing else switches in on him really, somewhat like Mega Mawile in OU. Here's to future users of Mach Punch!
Celesteela, just like Nihilego and Poipole, joins the ranks of the questionably-balanced Ultra Beasts! Here's some sets:

Mixed Wall (Celesteela) @ Leftovers / Metal Powder
Ability: Force of Attraction
EVs: 248 HP / 72 Def / 188 SpD
Careful Nature
- Heavy Slam
- Earthquake / Toxic
- Leech Seed
- Protect / Explosion
Even with no attack investment Celesteela's Heavy Slam is a scary prospect for offense to handle, with potential switch-ins Metagross and Tapu Bulu fearing Earthquake and Toxic, respectively. Leech Seed+Leftovers+Protect is Celesteela's form of recovery, while Explosion can be ran if you opt for Metal Powder to bring Raticate in safely further into a match. Also Force Of Attraction, while not aiding Celesteela directly offensive (Nihilego is already bopped by Heavy Slam), does ensure that Leech Seed and Toxic never miss :]

Automize Sweeper (Celesteela) @ Rockium Z
Ability: Beast Boost
EVs: 148 HP / 252 Atk / 108 Spe
Adamant Nature
- Autotomize
- Heavy Slam
- Earthquake
- Stone Edge
Z-Stone Edge always OHKOes AV Bulu after rocks, speed investmet outspeeds Scarf Raticate and everything below (so basically everything but Scarf Flygon). Heavy Slam+Earthquake hits everything else in the tier for at least (heavy) neutral damage. Watch out for Cofagrigus and defensive Cloyster, however