Have 2 tanks and 2 uv's. Never running without one, ever again, after a (bad) bacteria bloom nuked one of my tanks back a ways. I tried everything to stop it, but nothing worked. 12 hours after i installed the UV, my fish stopped dying and my water went from milk to clear.
When used properly they help. They will kill some free floating nasties - but not all, and not all nasty stuff is free floating. So there's that. They will also kill some good things like pods and a very, very small quantity of the bio-filter bacteria, though the vast majority of that bacteria lives in the rock and sand. So that much isn't an issue.
They are NOT a do all end all one ring to solve all problems item. You still have to QT, change water, not buy suspect livestock etc.
Just my 2 cents.
Oh, you do have to change the bulbs as recommended for it to work effectively. And flow rate vs lamp power matters when it comes to what the U V is or is not going to kill. It's something you have to figure out.