I think part of the lack of responses to this may be that many people, myself included, are having a hard time wrapping our minds around this tenebrium concept.
If I understand, even if you have 15 in 1 hand skill, you suddenly get no benefit from using a tenebrium dagger because of the material its made of? Your 15 points are irrelevant to your use of a 1 hand weapon made of tenebrium? And yet points in the tenebrium skill allow you to use a 2 handed tenebrium bastard sword with the same effectiveness as a tenebrium dagger?
If I am understanding this correctly, its one of the worst RPG game mechanics dealing with weapon skills I have ever seen.
If its a particularly rare material with specific usage requirements from a story line perspective, I can see there being penalties to your weapon relevant skill that might be offset by a talent or skill line. But no one skill should allow you to use completely different weapons with equal efficiency and no material should be so exotic as to completely negate extensive experience with a weapon.
I would very much like to see Larian or the mod community fix this (unless I am mistaken in my understanding).