What is more important: luck or fighting spirit?

Where did you get these formulas from?

All these formulas are in the game. You launch it, read the logs, and decipher the actual damage dealt when Luck triggers. You can take Naga for simplicity - she has constant damage.

Believe me, in my case (by the way, in that battle, the warriors who fell from the Knights' attack were mine), the Knights wouldn't have been able to take down those targets with a single hit. Believe me, even at a glance, it was clear that they couldn't.

If you ever need to delve a little deeper into this issue, you might change your mind. And if you don't, maybe that's for the best; it's not that important anyway. I wasn't expressing my own opinion, but my observations... :)

It's a pity that such units are very vulnerable. I mean, there are artifacts that reduce the morale or luck of the enemy hero, and then these abilities become just a waste of skill slots.

But agree, when these abilities are present in the enemy, and you have the opportunity to neutralize them, then it's not a pity, but very good. So, there should be a countermeasure for every strength to maintain the game's balance of power.