Yeah, UV units are one of those things that you never "really know" if it's working.
A lot of UV loyalists will talk about how they've "Been running one forever and never had an ich outbreak so they are great." The thing is though, you never really know whether or not the UV unit had anything to do with it.
I have special paint on my house that repels elephants. How do I know it's working? Well... you don't see any elephants around... so....
IMO UV units are great for two scenarios... multi tank systems and ponds where free floating algae blooms can be impossible to control by other means.
But they are expensive to buy, require careful design considerations to deploy properly, and the lamp changes really eat up cash.