For one, you have to consider both Hippopotas and Snover - these two, statswise, would be considered NU pokemon no doubt (without their ability) - Good luck actually trying to do anything with them, you will essentially be playing 5 versus 6 the whole game.
Snover probably, Hippopotas not so much. 340/280 HP/Defense may pale in comparison to Sandslash, but is still decent enough to take a couple of hits on the physical side, and still has reliable recovery, Stealth Rock and the ability to PHaze. STAB Earthquake off 181 Attack is also still reasonably powerful, although unSTAB'd attacks are now out of the question. Nothing amazing, but far from dead weight after weather setup.
Furthermore, good luck keeping them alive - since Sunny Day and Rain Dance users are widespread in UU and far more common than in the above Tiers, all it takes is one Victrebell to use Sunny Day once after your Snover has been blown to hell by Spearow's Peck and your whole team is essentially fucked.
True, but the fact that permanent weather is available basically forces teams to either always carry weather changers, abuse the infinite weather themselves, or base their entire team around countering Sand/Hail teams. All include centralization of some sort, which is what UU should be looking to avoid.
There may be one or two pokemon, as previously mentioned, who, especially in hail, outclass the rest of UU, but it could be easier to just put those few in BL. Also, I don't quite see pokemon which become overpowered by the existance of Hippopotas in UU.
Cradily and Shuckle become virtually invincible on the special side, Cacturne becomes annoying as fuck as a SubDancer, whilst the likes of Probopass, Golem and Magcargo also get a much welcome boost to their special defense, although still have major weak points. Just a few examples off the top of my head.
EDIT: What Shiny Oddish said also. Need to type faster.