30k pen, 6k damage or 30k-40k max stat and 100% crit bonus damage will do that to ya. And that's before you add in the multipliers like Berserk, Incap, and Champion Points. Add all of that onto the single best scaling attack in the game and you get... 20k damage? Is that all? Really?

25k resist your also missing out on some mitigation from armor, as your 8k short of the armor cap

33k is 50% mitigation

16.5k armor is 25% mitigation

at 25k armor your sitting at around 37.5% mitigation, so thats 12.5% mitigation your missing out on

though with the amount of pen one can get, armor itself doesnt help a ton unless your really stacked on it

also sounds like crit resist is kind of low, or missing other mitigations (major/minor protection, etc)

Yeah it's totally a learn to play issue and not a balance issue. That skill is completely fine and not overtuned. It should be buffed if anything, 20k isn't enough. We all know ZOS hates nightblades.