As some anons suggested here, visiting the UBs' home worlds might mean that since we capture them before they cross dimensions, they could be obtainable with another ability than Best Boost. What ability would fit them ?
Pheromosa : Queenly Majesty
Buzzwole : Intimidate
Kartana : Hyper Cutter
Nihilego : Levitate
Celesteela : Flare Boost ?
Guzzlord : Gluttony
Xurkitree : Lightning Rod
Burst : Flash Fire ?
Assembly : Regenerator
Adhesive : Gooey ?

Any idea ?