i install them and i believe they do a fair job, especially here in FL with a lot of humidity and guess what, yes mold and mildew. evaporators coil must be new or just deep cleaned, only then the UV light will keep the coil clean.
and, i install them on the wet side of the coil, where should be installed, so they keep clean coil and drain pan as well
for your info, i wrap all wires with aluminum tape so they don't get damaged from UV rays.
my 2 cents