Considering how powerful they are, and how Silvally's movepool is bad, he might just fall flat to them. I guess Fairy Silvally can get past Guzzlord, at the very least.
Silvally would actually 1v1 every single Ultra Beast with either of the memory
Like its basically like this
Pheromosa - 95/95 is not getting one shot by non SE anytime soon
252 SpA Life Orb Pheromosa Focus Blast vs. 252 HP / 0 SpD Silvally: 138-164 (35 - 41.6%) -- guaranteed 3HKO
252 Atk Life Orb Pheromosa High Jump Kick vs. 252 HP / 252 Def Silvally: 117-138 (29.6 - 35%) -- 15.1% chance to 3HKO
Buzzwhole - Flyvally LOL. Also
252+ Atk Choice Band Buzzwole Stone Edge vs. 252 HP / 252 Def Silvally: 308-364 (78.1 - 92.3%) -- guaranteed 2HKO
Guzlord - Fairylly LOL
0 Atk Guzzlord Heavy Slam (120 BP) vs. 252 HP / 252 Def Silvally: 144-170 (36.5 - 43.1%) -- guaranteed 3HKO
Nihilego - Steelvally LOL
Kartana - Firevally LOL
252+ Atk Choice Band Kartana Sacred Sword vs. 252 HP / 252 Def Silvally: 170-200 (43.1 - 50.7%) -- 3.9% chance to 2HKO
Celesteela - This one depends on Celesteela EV Spread, but generally speaking Celesteela probably won
252+ Atk Choice Band Celesteela Earthquake vs. 252 HP / 252 Def Silvally: 248-292 (62.9 - 74.1%) -- guaranteed 2HKO
0 Atk Silvally Multi-Attack vs. 252 HP / 0 Def Celesteela: 182-216 (45.7 - 54.2%) -- 48% chance to 2HKO
0 SpA Silvally Flamethrower vs. 252 HP / 252 SpD Celesteela: 146-174 (36.6 - 43.7%) -- guaranteed 3HKO
Xurkitree - Funny enough this one had a chance to beat Sil
252+ SpA Choice Specs Xurkitree Hidden Power Ice vs. 252 HP / 0 SpD Silvally: 280-330 (71 - 83.7%) -- guaranteed 2HKO
0 Atk Silvally Multi-Attack vs. 0 HP / 0 Def Xurkitree: 246-290 (80.1 - 94.4%) -- guaranteed 2HKO
So thats 5/7 out of the UB, thats good enough
Point is 95/95 isn't a mediocre defensive stats and Silvally's movepool is actually godly for what it is
252+ Atk Choice Band Pheromosa High Jump Kick vs. 252 HP / 252+ Def Silvally: 271-319 (68.7 - 80.9%) -- guaranteed 2HKO
252+ Atk Choice Band Pheromosa High Jump Kick vs. 252 HP / 252+ Def Skarmory: 207-244 (61.9 - 73%) -- guaranteed 2HKO after Leftovers recovery