Definitely. Not much debate there. It has phenomenal stats which are very well rounded. Base 160 Attack with 100 Speed to back it up? That's hard enough to deal with. Add in tremendous HP and fantastic defenses and you have yourself a monster that is banned to Ubers before you can say "Regigigas is in PU".
That said, it would still see little or no usage, owing to its poor movepool. Sure, there's diversity, but not the brute force one sees in Geomancy Xerneas' Moonblast or M-Rayquaza's Dragon Ascent or V-Create.
Knock Off, Thunder and Ice Punches, along with Earthquake would dominate most standard sets with some variation from time to time, in the form of support with T-Wave, or set up sweeper with Rock Polish (late game demolisher) or the occasional Stone Edge/ Heavy Slam in place of either of the two punch. Maybe even Zen Headbutt. See, they're all good moves, but not strong enough by themselves.
Given its typing, it's not as threatening as others in Ubers, but nevertheless will not be allowed in OU.
One thing to keep in mind is Z-moves. It can utilize Z-moves to some extent but to be honest, others can too, and hence it's not a game changer.
As for threats, anything which is slightly faster and can hit harder fits the bill. M-Gengar with Shadow Tag + Destiny Bond is basically the de facto counter, providing a fail safe like no other. Further, Burn massively cripples the Continent Pokemon and renders it useless for the rest of the game. Sure, Rest can be used, but Ubers is generally too strict with such passive styles of playing.
To sum it up, owing to Stats (BST), Regigigas would almost certainly (over 90% chance, which is an arbitrary estimate, but I mean very likely) be banned to Ubers. Unfortunately, Ubers wouldn't treat it kindly, making it sink towards the lesser used side of the same, along with Arceus Grass or something obscure.