For a slow pokemon with good defenses to be weak to stealth rock isn't a good thing, but with defog that is mitigated somewhat. It is weak to Ground/Rock but they are mainly physical moves and pokemon that get STAB on those moves are weak to the best water type attack in the game, which only this thing gets. Said ground/rock 'mons can revenge kill, but will need strong offenses to crack Volcanion's defenses. It is also weak to electricity, which means Rotom-W and Thundurus (both forms) can also do significant damage, but I don't think Rotom-W can take it out w/o specs, while thundy can't switch in. That gives it three weaknesses, 2 of them do nothing to a similar pokemon, Landorus-T, which laughs a most ground and rock types and can beat electric types. So far this pokemon looks like it will be a driving force in OU this gen, capable of running defensive, potentially offensive pivot, and choice sets all seem to be possible this likely addition to the tier.