RU dosen't have to deal with Hail, Chansey, Staraptor, and the other myriad of UU threats.
RU is a completely different metagame from UU, and what is allowed in it is based off of what happens in UU.
Attempting to base if Hail is broken in UU on if it's broken in RU is like saying Imposter Ditto wouldn't be broken, because it's power depends on the power of the teir it is in [Despite it being the last word in momentum, and literally HURLING your momentum back at you]
So saying that if Hail is not broken in RU, it's not broken in UU is stupid, especially as many Hail abusers are UU [Froslass, Mamoswine, apparently Nidoqueen is a Hail mainstay], as well as Abomasnow.
I'm not supporting Snow Warning being banned, however, and, frankly, I do think Froslass and Mamoswine are the problems, if any exist with Hail at all. [Good sign is how much more they are used than Abomasnow] I was just pointing out the flaw in your argument.