UV is highly effective in eliminating Green Water Algae (GWA). It is also used as a prophylactic in the tank as data have shown effectiveness against certain free floating pathogens but it is very difficult to quantify the effectiveness. As mentioned by Edvert, it will not save you from silly mistakes. It is also at odds with the general requirement for high flow of a highly lit CO2 injected tank. In that case it should be employed on a separate low flow water circuit because it's effectiveness depends on the exposure time (referred to as "Dwell Time") of the radiation on the pathogen.
Cheers,