Nah, I remember back in ORAS there was an Assist strat for multi-battles called Voidcats. One person brings a Liepard with Assist, a Smeargle that knows only Dark Void and a third Pokémon that knows moves that Assist can't select. It was fun to use because of Liepard's Prankster ability. Thing is, Dark Void is Darkrai's signature move. Unless they changed the way Assist works in Sun/Moon, I'm pretty sure it can still use signature moves.
(Well, now it can't use Dark Void because only Darkrai can use Dark Void but my point still stands.)