i think what i'm trying to say (which even the piercing player themselves said to me outright) is that the "Find the Weakspot" felt too easy to use with no real consequence, even with the fact you can't crit and have a higher chance of failing, it don't feel like a special attack if you can do it repeatidlly. The others are more situational but can swing the tide of battle (like disarming an evil knights enchanted sword and stealing it), but going "i try and stab the weakspot" is always something you can do, and if your skill is high enough you can do it easily, which can make armor feel useless. (And if you can pirece through it, why would'nt you?)This isn't the GM Only section so to avoid spoiling, but what stopped the monster from being smart and attacking the piercing enemy, forcing him to either just take the damage or to use his action to parry and thus stop him form attacking? But lets not go into detail in that scenario here. Sure it can be a good choice, but I haven't played enough to judge if just this special attack is OP or not.
Hence i suggested it being turned to a heroic ability, or maybe instead it pirces through half the armour rounded up, so you can't do it as easy.