What I can tell you is only my experience, I wouldnt go out and buy one based on my one action, it is up to you.
If you google UV filters, in many articles it does state that it kills diatoms in the water. It will not kill EXISTING diatoms on rocks, etc since it is not flowing in the UV filter.
Further, a successful UV filter has to do with the wattage of the light and also the flow through the filter. The problem with UV filters which are built into filters is that they are not matched together. My UV light is 9watts and puts out less than 50gph, which is very slow flow. With cheaper units like the filters sold on Ebay, the UV light is just a dressing. It is not designed properly and/or the flow is too fast. There are also other issues with the spacing, a good read on this is here
I have a LOT of light above my tank. When I ran the UV light, it lessen or I didnt even see any more blotches of Brown Algae in my tank. With my lighting, I can literally see it grow in a few hours.
Ironically, I was doing a water change, and air got into my new UV light, and the pump wasnt pumping water. That day, I did get more brown spots.
The next day, I fixed the issue, released the bubbles, the UV light worked and that day, I got less or no new spots.
Is it scientific? No, coincidence? Maybe.
But what I do know is my friend with a large bow tank moved some plants around, he run 2 filters, and even ran a polisher on a 1200 powerhead, and no matter what he did, after a week the water did not clear up. He went out and bought the same UV light, and the VERY NEXT DAY, the water was CRYSTAL CLEAR.
So if anything, maybe it can keep the water clear.
My friend also commented that he had spot dust algae on the glass, he would wake up each day, scrap it off, and ever since he been using the UV light, no more dust algae on his glass.
Just our 2 experiences, but it has been positive for us, but there may be other factors also, who knows.