The biggest problem IMO is that Tenebrium weapons basic damage is higher than equal other level weapons.
Then it also got Tenebrium damage which most enemies cannot resist.

Of course, slightly balanced by not having bonusses (I have yet to find a proper enchanted Tenebrium weapon). Overall, weapon skills are useless (IMO) I noticed in the beta so put no points in them at all, so it's not really an issue for me 0_o

I think the best solution would be to have elemental bonusses boosted by skills. So the +Tenebrium damage by Tenebrium, and the regular damage by the regular skill. Then other boosts like Earth, Fire, Air etc. are also boosted by their suspective tree. I don't think it would be that overpowered, and would only give a minor boost to range/melee + wizard specifications. And might help a bit with Staff of Malgus for mages.
Overall, nothing I would see would be wrong by such a change, but I'm open to discussions if you see a point.

@ Hiver; Indeed, higher leveled enemies might get major to-hit penalties, but those are very easily overcome. Frozen, Blinded, Knock-down, Petrified... and many more make all hits automatic hits.