Hail is NU because:
- Only two abilities to abuse: Snow Cloak and Ice Body
Ice Body only have use on Walrein for it's Substitute + Protect stall, and at some degree for Regice, but even with Ice Body, Regice is still unnable to wall things that he wasn't able to stall before, so, it's just to mantain him more healthy for the course of the battle. That pair is just for accesory stall on balanced teams, because you can't make solid stall teams with the current hail, and offensive teams just don't have any use for Stallrein, or to have a somewhat more healthy Regice.
Snow Cloak by other hand is more decent, but it only have 3 viable users: Articuno, Glaceon, and Beartic. Articuno can use it on more offensive teams to make a sort of Offensive Stall set, pretty effective, but nothing unbreakable, and Articuno still have it's Stall set with Pressure, too. Glaceon have a 20-38% of chance to survive a hit and deal an extra K.O or deal good damge before faiting, but that's all. Beartic can make some use of it on Balanced Sets, but Beartic on Rain with Swift Swim outclass any offensive set of Beartic on Hail. Oh, and also, this ability is already banned on higher tiers, so, it's banned here, too, so, you're not even having this option. (We all know this was banned because - yes - , so, I'm mad about the reason behind it's ban).
Summary: Those abilities are gimnicky at best, except for Walrein, and he's not that great.
- Not important secondary effects: 6% residual damage and more viable Blizzard use
Let's see, 6% of damage just nullifies the recovery of Leaftovers or make a somewhat harder time to some (bad) walls with just Rest to survive, break Focus Sashes and Sturdy users, and make a somewhat benefit to archive some extra K.O's for offensive teams. In other words, it just do the same as Stealth Rock do; of course, you can combine both, but with Stealth Rock available, it just do an extra help with the first and last point, not bad, but far to be broken just because it.
By other hand, the unique reason to use Hail for start of: you can change that Ice Beam for Blizzard on any Pokémon and set that use Ice Beam. Let's see the difference on power:
95 > 120
Yes, it's only an improve of 22% of power, in exchange for some things: less PP, not a problem at all, but you'll have harder times with the walls, and... if the weather is changed (weather teams on NU are not so uncommon), you'll have a move with 70% of accuracy, ugly. Still, if you combine it with the 6% recoil, you'll have better times with balanced and offensive sets of those Pokémon. Nice, but it's unique powerful abuser is Rotom-F, who had the strongest VoltBeam combo of the tier (and at some degree, of the game), Rotom-F can be a real pain to deal with it... if it weren't for it's horrible speed, of course, you can patch it's speed with Choice Scarf, but you can just use faster Choice Scarfer than Roton-F to detrone him, or revenge kill him with priority, or just use prediction: it will just Thunderbolt/Volt Turn? Chose to a Ground-Type (you should have one) Pokémon and force the switch, it will use Blizzard? Choose your favorite Special Wall, or just any Fire/Water/Ice/Steel Pokémon and prepare for the switch. Roton-F without Choice-Scarf is really easy to destroy, just beware of Thunder Wave and Trick.
Even if Rotom-F is too overpowered for the tier (I'm not having troubles to take care of him, but some does, it seems), just ban Rotom-F on place of Snover, derp.
Summary: VoltBeam is a cute combination, and Blizzard is awesome, and the recoil is of help if you're not carrying Spin-Blockers, but even then it rarely allows you to deal extra K.O's, or to use more effective tactics than others.
Oh, yeah, at difference of weathers on higher tiers, Hail on this tier have a drawback:
- Snover sucks as hell, and die by the hands of everything, making it a 5vs6 match
That's all, you're purchasing minimal effects (22%+ of power to Ice Beam, 6% of residual damage - to your non-ice-type Pokémon, too! -, some viable sets) for making the game an effective 5vs6, and even then, you'll have not benefits on a 5vs6 if your opponent is carrying an weather move, great. We all knows that hail seems more stronger that it is really cuz Rotom-F, but Rotom-F is with or without hail a powerful Pokémon, and if hail is the difference that make a borderline Pokémon into UU, Rotom-F should be banned on place of Snover (but I don't think Rotom-F on hail is near to be broken, anyway).
Of course, you have some (and better options, at my sight) for 5vs6 matchs, like Tailwind suicide, and sweep the opposition with Choice-Bands and Choice-Specs, or just Double-Barriers with Light Clay + Status Move lead, it's just piece of cake to set and sweep with the Double Barrier, and more if you can paralize, burn, or sleep one of the opponent's Pokémon. For example, you can just do Tailwind suicide, use Rotom-F with Ice Beam and Choice Specs, and you'll deal more damage than Rotom-F Choice-Scarf with Blizzard on Hail with any move (and you'll have more speed, too).