That actually makes me think. The other Ultra Beasts probably have individual abilities beneath the effects of Beast Boost, given said ability stems from the other worldly aura. I could see Kartana having something like Sniper or Technician given it small size and sort of origami/samurai motif gives the impression of delivering few precise cuts for damage.
That said, it makes me wonder if Beast Boost is the generalized ability, or if it just happens to be an approximation of what their individual abilities would be in some fashion.
Maybe this could be the justification for Hidden Abilities on the UB's compared to Legendaries just sort of "having" them without any real reason. A Pheromosa introduced recently as in Sun/Moon would have Beast Boost, whereas one that's been hanging around for a while would have a different ability like Natural Cure, MOld Breaker, Quick Feet, etc. It'd be an interesting way to justify UB appearances in games post Alola.
My guess for possible "natural" Abilities (of existing Abilities, and nothing that'll make it too broken like Huge Power Buzzwole):
Nihilego: Clear Body, Insomnia, Levitate, Liquid Ooze, Trace, Unaware, Unnerve
Buzzwole: Compound Eyes, Defiant, Guts, Hustle, Inner Focus, Intimidate, Mold Breaker, Moxie, Sheer Force, Tinted Lens, Vital Spirit
Pheromosa: Dazzling/Queenly Majesty, Inner Focus, Limber, Mold Breaker, Oblivious, Run Away, Shed Skin, Shield Dust, Tangled Feet, Wonder Skin
Xurkitree: Battery, Galvanize, Illuminate, Insomnia, Lightning Rod, Static, Volt Absorb,
Celesteela: Aftermath, Flare Boost, Flash Fire, Heatproof, Heavy Metal, Overcoat
Kartana: Dry Skin, Infiltrator, Insomnia, Levitate, Light Metal, Mold Breaker, Sniper, Super Luck
Guzzlord: Gluttony, Hyper Cutter, Immunity, Intimidate, Multiscale, No Guard, Pickup, Pressure, Strong Jaw, Thick Fat
But considering how Solgaleo, Lunala, and Necrozma have unique Abilities, and considering the description of the Ultra Beasts, they'd probably also have unique Abilities:
Nihilego: Ability version of Swagger upon entering battle, reflecting how its poison works; Neurotoxin?
Buzzwole: Honestly Beast Boost/Moxie isn't far off what I would give it. So I guess a copy of Moxie with how it would increase its strength; Fluid Drain?
Pheromosa: Inflicting Infatuation upon entering battle (doesn't matter if its genderless); Pheromone?
Xurkitree: Special Attack version of Speed Boost; Power Route (get it? "Route" as in the computer command and "root" as plant roots)?
Celesteela: Fire-type version of Steelworker; Combustion?
Kartana: Cutting/Slashing move version of Iron Fist; Fine/Sharp Blade?
Guzzlord: An Ability that makes all Pokemon in battle lose their item; Vacuum Stomach?