I have a small in-tank unit I toss in when I feel it is worthwhile.
I mostly view it as useful for controlling green water. While may help with diseases, it will only control infections that spread primarily by hanging out in the water column for long periods of time.
Anything that can be spread by fish-to-fish contact, such as during feeding, is going to completely bypass the UV unit.
That said, the UV unit certainly won't hurt anything, except maybe weakly chelated iron fertilizers.