Is it generally better for a weapon to have a UV bonus for things that are weak to it or resistant to it?
For instance, I just got a Voltech Alchemer with high dmg vs Gremlins. Gremlins are resistant to elemental. Is that considered a good UV (since it overcomes the weakness of the gun) or a bad UV (since it's not playing to the gun's strengths)?
UVs cannot negate resistance by any significant measure, so a UV Damage Bonus vs Gremlin High on a Voltech Alchemer is completely pointless. Here is an example using the Ionized Salt Bomb which is half Piercing but has a natural Damage Bonus vs Slime High (which resists piercing):
Tier 3, Depth 19 Damage
Beast/Fiend (weak to piercing) 166
Gremlin/Undead (neutral to piercing) 121
Construct (resists piercing) 27
Slime (resists piercing, but has damage bonus high) 31
So the damage bonus added a whopping 4 damage per hit. The damage to slime is still about 4 times lower than a neutral creature at that depth. Meaning that if someone is using the Ionized Salt Bomb on Slimes they must be really desperate and ill prepared despite the damage bonus.
The Damage Bonus UVs are multipliers, so if it starts off with very low damage it'll still be very low damage. That is why UVs should focus on the strengths of the weapon, or the neutral damage at the very least.
If someone got a Damage Bonus vs Beast High on the Ionized Salt Bomb then the damage would be about 191, or an increase of 25 hit points... very respectable and could make a difference on the battlefield.
If you only use that gun as a sidearm then it's kinda good because it'll be more effective but if you are a gunslinger with many guns it would be better to have one gun specialized for each enemy type instead.
The UV won't negate 100% the reistance the enemy has.