Testing has shown it's not that bad. SE attacks still fuck it up good, and his movelist only has a few moves that that Ability would benefit.
Its beneficial moves, like Growth, are reversed as well, so all of its boosting moves are fucked over good. That means no Coil to boost Attack, Defense and Accuracy. All those moves are now useless, and it could use them a lot.
So really, it's not that bad. It can hit hard, but it's still kind of fragile. As a Serperior (final stage evolution), 75 base HP means that even with 95 base defenses, it's not going to stand up to much damage. STAB Ice Beam = GG.