I've been testing North Pole as well, and I think I found its niche, which I'm going to call "vertical facetank". It basically gives you obscene levels of DPS in a large vertical area, but the drawback is that you can only dodge by moving up and down. It is actually extremely good against specific bosses when using a full Warding loadout. (You can't be invincible, but you can get pretty damn close!)
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0:24 Golem in Master mode using this loadout. In comparison, Starlight is a 0:26 kill using the same equipment.
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0:44 Empress of Light in Master mode, far sloppier kill but it's kind of amazing what you can get away with when you have 140+ defense. She's only in the snow curtain half the time but when those snowflakes are hitting, she gets ripped up like tissue paper.
Tried it against Duke Fishron as well, my conclusion was "don't even bother".
Moon Lord? If you set it up before he spawns, you can bring the top eye down to 50% before the first deathray comes out, which was worth it, I think. Second deathray comes out at 40 seconds, and North Pole can finish it off then. And after taking down the other eyes, I was able to get this far with the North Pole before dying (granted I played pretty sloppily, so there is a limit to how much defense stacking will let you get away with).
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(in retrospect i should have kept the squire's plating)
In conclusion, North Pole is kind of
when you use it the traditional way, but does really well against specific bosses during specific phases using specific loadouts. I do wish it were less specific, but I don't think I'd agree with a generalized buff.