Main issue is cost, due to size. Most people grossly underestimate the size of UV they need. IMO, something like a 125gal, needs at a minimum, a 30+ watt UV unit. Then, based on the hours used, the bulbs need replaced about every 9 months. Especially considering most are running 24/7.
That said, I used one on my 125gal. From sheer math, you can decipher that it can't help with parasites. I did have a hair algae outbreak, the UV didn't help with. I, however, always seemed to have crystal clear water, that I think I can say the UV helped with.