By "areas with less protection" you mean head I guess, on those with LA or MA who try to keep malus at bay by not using helm, which also refers to stun. Having a coin toss every two hits to stun the enemy could be overpowered but I wonder if problem is the 50% chance or the stun itself.
What I would try to avoid is this scenario: "It's only worth using aim to the head, and in case of target not wearing helm (being stun a nice random bonus)". That is a fair call, no helm no head, but the other effects should be meaningful too in order to have combat tactics.
So we first need to ask ourselves if that 50% would be interesting for combat effects in general, and in case of need tweak down those that would become overpowered. Stun, for instance, could have less duration and become more like an action breaker instead of a kipee-like target disabler.
What I would try to avoid is this scenario: "It's only worth using aim to the head, and in case of target not wearing helm (being stun a nice random bonus)". That is a fair call, no helm no head, but the other effects should be meaningful too in order to have combat tactics.
So we first need to ask ourselves if that 50% would be interesting for combat effects in general, and in case of need tweak down those that would become overpowered. Stun, for instance, could have less duration and become more like an action breaker instead of a kipee-like target disabler.