Interesting question! I think your line of thinking is not wrong, but some counter points worth considering:
1. Some enemies are just being asked to be head-shotted. Up until you reach lategame, plenty of enemies wear little to no headgear. The weasel guy in the tutorial oddly is one of them – don’t think I’ve ever seen him wear head armor. In the late game though, only Sergeant, Berserkers and geists come to mind. All are very deadly though, geists die in one hit anyway.
2. Nazghoul thingies never have armor. Hitting the head is always better. Werewolf armor is arguably weak enough to make the same reasoning.
3. Sort of related, flails and axes are generally useful for the Ancient dead’s frontline troops. They have shield wall every turn, have infinite stamina and maybe even have shield expert. However, their shields are shockingly fragile for late-game troops so one greataxe specialist smash always breaks it. You’re meant to either break the shields, use flails to ignore the shield defense. I think it’s probably better to use reach weapons/greatswords to hit the priority targets behind them though.
4. Duelist builds. I wonder how much damage we’re looking at for an axe head hit from a duelist? What if the brother has the brute perk? The idea is to kill the enemy under the armor before the armor breaks, so head hits may be always welcome. Alternatively, 2H weapon may kill before armor is destroyed if you keep getting head hits with a brute, particularly 2H hammer.
I think 50% bonus to damage applies to both HP and armor.