I have uv lights on two tanks and they have done nothing for cyano that I have. Actually I cant see where they have done anything at all TBO. I've varied flow rates and still have as much if not more nuisance algae than before. I've been running them for almost a year and probaby will pull them when the bulbs need replaced. I knows lot of folks run then but they have done nothing for my tanks that I can see. They are not undersized for the water volume either.
EDIT: I can't comment on disease control as I have none at this time and all new additions are run through QT