I don`t know many would go for damage against beast, fiends, etc and much more would go for just a overall damage bonus
So my suggestion is to change the uv bonus instead of towards a single enemy but to boost the overall specific damage that a weapon does (piercing, elemental, shadow)
The damage boost uv compared to the over damage uv:
the piercing uv would do better damage output on fiends and beast
slightly less on undead and gremlins
the elemental uv would do better damage output on constructs and undead
but slightly less on slimes and fiends
the shadow damage would do better damage output on slimes and gremlin
but slightly less on beast and constructs
(there is proberly many flaws and possibly lack of research in my suggestion but i just wanted to give a thought out and see how it goes)
Interesting concept, but how would it work for UV's that arent on that weapons dmg type. For instance, my voltedge has a fiend vh UV, would that make it less effective on contructs? and better on beasts? You have to consider that UV's dont always happen on the same dmg type as the weapon, its actually rare i think.