>>2746239If I understand the mechanic correctly, its 8th level ability would not prevent the full resistance from applying to the cold damage half, so if you did 60 damage total against an enemy with 30 cold resistance it would still be reduced to 30 with or without the ability. With that said, you could use it to save a mythic ability on Ascendant Energy if you want, you already get vulnerability curse to remove energy resistances.
Compared to Elemental Witch (you have to take this to get the Winter patron), you would stop scaling the Elemental Blast they get in exchange for getting full Hex progression and the Winter Witch bonuses to spell DC. It's probably a decent tradeoff, Icy Prison and Polar Midnight are both very good control spells so increasing their DC is going to be a good investment, and you can fill time with empowered or Maximized snowballs without getting in the way of your 5th level slots for Icy Prison instead of using Elemental Ray. The wiki lists the 9th level spell of Winter patron as Polar Midnight, but since they already get that I'm guessing it's actually Mass Icy Prison like the shaman Frost spirit. If not you could just take a Loremaster level to get it.
Most Shamans can take Frost spirit, so you're not getting much of an advantage on hex progression. There isn't really a subclass that would benefit Winter Witch over the base class, so everything I said for Witch mostly applies to Shamans.