Droppin' in. I'm with Mashew here, although that might be inevitable, sadly...
The way I look at it is the movepool: yeah, it sucks - it doesn't even get Earthquake. But it's got some nice moves going for it. I've found that it's an incredible user of a sub-roost phazer set - the best, I think. Freeze Shock - yes, the move sucks. But who needs that when you have access to another version of Mold Breaker, and Dragon-Electric neutral coverage? This thing plows through walls better than Haxorus, Excadrill, and even TR Rampardos, while even having perfect neutral coverage barring Excadrill and Magnezone.
Its base stats are the reason why it's so scary. Base 170 Attack, like I said before, plows through walls in combination with its Ability. It also has 120 base Special Attack so it's ridiculous from that side of the spectrum. 95 base Speed is very viable for it to be a sweeper due to its awesome bulk. Oh yeah, the bulk. One does not simply say no to 125/100/90 defenses. Adding on top of that a resistance to Water-, Electric- and Grass-type moves, making this a total shutdown of Pokemon that attempt to stop rain teams. It takes a lot to get through to it and OHKO it, and if we're talking a neutral attack is not going to do anything to it that it can't heal off without investment.
The vulnerability to Stealth Rock...yeah, it's not that big an issue or else Volcarona wouldn't be OU, maybe not Dragonite either. Rapid Spin support is easy to find with Tentacruel, Starmie, Excadrill, Cryogonal, etc., that clear these things away - in the case of the latter two, they between them have no problem killing off the setters (Jirachi, Skarmory, blah blah). Give it the right team support (ideally RS + rain) and you'll see the reason why we leave it in Ubers speak for itself.