Omg Auroura Veil and Snowstorm's effects together will be crazy!
They are, but the problem is basically that...
Ice still sucks defensively. Aurora Veil was actually mostly used for non-ice pokemon and the Hail's only purpose was to enable it in gen 7/8.
Arctozolt (and to some degree, A-Sandslash and Arctovish in lower tiers) were the only real "hail abusers" that actually wanted to use Slush Rush, and even then, they still wouldn't exactly take hits well despite aurora veil due to the horrible defensive type.
Unless this gen blesses us with some strong slush rushers or buffs the ones we have (tecnically, I guess we did get a new one, tho we don't know its base stats), it's one of these "nice on paper but useless" buffs because you're still not going to use ice types with hail in order to actually benefit from it.
Sand is a bit of different story since Rock types tend to both be bulkier and their defensive type is bad but not THAT bad, and almost all rock types have naturally high defense so the spdef buffs just patches it up.
A-Sandslash and Arctozolt are still getting murdered by a random stray flamethrower or close combat, even with this buff.