The disadvantages are they don't really do much, and they drain your wallet of money.
They have nada to do with filtration, and the ones 99% of hobbyists use aren't likely effective against the common parasites they're used to combat as 1. the hobbysist units aren't powerful enough to kill ich and 2. you need to change the bulbs very, very often, and clean the tubing.
So if you're looking to reduce water changes money spent on a UV is about as useful as spending it on beer.
What do you think they do in the 'for' section?