This would makes maces completely useless to player. Enemies do have perks and you have no way of checking what they have. So you will never be able to rely on stun.
I already find maces sub-par. Orcs are not stunnable, undead ignore fatigue damage, goblins are hard to hit and die before fatigue becomes issue for them. So it’s basically anti-human weapon, which is too narrow to be worth specializing in.
since you mentioned the undead, i really hope mace could do extra damage to those skeletons.
but what if puncture only works well on stunned units, then mace or what ever could stun a guy will be worthy specially in the early game, not mention that you no longer have to worry the fact that in current version of the game, if you let your 2handed heavy face a bandit marksman who has switched his weapon to a dagger, you heavy guy due do lack of shield protection will be stab easily and get deeply injured or even worse.