I don't think you should be relying on Tentacool that much for taking Fighting- and Fire-types all that much. In particular I'm referring to Sawk and Emboar who both utterly obliterate Tentacool. Hell even Swords Dance Samurott (who has more liberty in the metagame of Snover and crippled Amoonguss with unreliable Synthesis) can defeat Tentacool as a Swords Dance Waterfall does tons to it and Tentacool's Giga Drain does awful damage.
- 252 Atk Choice Band Sawk Close Combat vs 252 HP/200 Def Eviolite Tentacool: 50.7% - 59.86% (2 hits to KO)
- 252 Atk Choice Band Emboar Flare Blitz vs 252 HP/200 Def Eviolite Tentacool: 50% - 59.15% (2 hits to KO)
- 252 +2 Atk Life Orb Samurott Waterfall vs 252 HP/200 Def Eviolite Tentacool: 50.35% - 59.15% (2 hits to KO)
Anyways now that I have been playing around with Hail for a bit I can give my opinion about it. The Hail provided by Snover is very powerful as it makes Pokemon such as Rotom-F and Glaceon absolutely incredible. I think Snow Warning will fit just fine in NU for a few reasons. The benefits to having Snow Warning are that the hail decreases the effectiveness of Synthesis/Moonlight, Snover is a counter to Ludicolo and Special Samurott, and that the Hail buffs Blizzard to 100% and gives Glaceon, Rotom-F, and even Walrein a powerful STAB move. By limiting Synthesis and Moonlight recovery it puts a hamper of Amoonguss and Musharna, two powerful forces in metagame. In a way it sort of "balances"(not that they were broken before) them out further such as in the latter's case where teams without Absol or Skuntank were generally deemed weak to Musharna. By limiting Musharna's recovery it relieves a lot of pressure on teams as Musharna cannot abuse its amazing bulk the same way anymore as it will now only heal 25% per pop. Ludicolo's counters in there metagame are really just Mantine, Regice, and Snover, and now that Snover has more uses including cancelling out Ludicolo's Rain, it will also put a damper on another powerful force in the metagame. However the fact still stands that you are forced to use a Snover if you want to get the violent snowstorm rolling. While Snover has a cool niche in beating Ludicolo and Gorebyss, it's still just acceptable and not exceptional as Stealth Rock and lack of recovery outside of Leech Seed wears it down constantly even against the previously mentioned Pokemon. If you want to abuse to Hail you'll bring a Snover and 1 or 2 abusers which compounds the weakness that the Ice-type Pokemon share. This makes you naturally weak to Emboar, Braviary, SD Samurott, and Sawk; which are already not easy to deal with in the metagame.
Duosion is already showing considerably more usage due to the fact that it has Magic Guard and Recover over Musharna. However Duosion is no Musharna, and their bulk is not really comparable. Musharna's bulk is considerably better than Duosion and because of this even teams without Skuntank and Absol don't really have as much trouble with Duosion as they would with Musharna.