I have a fowlr, Does the negative aspects of a UV still pose a threat?
IMO yes. It too will give you a false sense of security for some of the same reasons as the diatom. If the ich doesn't pass by the light (when the light is operating at full strength i.e sleeve is not dirty, water isn't moving too fast)..then it won't kill the ich.
What a UV does is kills bacteria. Indiscriminately..it doesn't care if it's good bacteria or bad bacteria, so yes, it does matter in a FOWLR too, as they keep your tank cycled.
Regal's post did a great job pointing out the pros and cons.