@Tyhan
Actually, there are enemies in the game who are immune to piercing but not crushing/slashing. Just not a lot of them...

@Gyson
That's not a bad idea. Since Tenebrium weapons are supposed to be more beefy than common weapons, the penalty makes sense.

At this point, I'm OK with just taking out the Tenebrium bonus damage all together. They can make it strictly a requirement for using a Tenebrium grade weapon and let the original weapon ability apply the damage the bonus instead.

@thrakkemarn, ScrotieMcB
I was pretty surprised when I first noticed this and I'm glad I'm not the only one. Hopefully the devs get a chance to rework the ability before we resort to mods.