I recently added a new canister filter with a 9 watt UV and all was great for the first few days, tank was very clear but then after day 4 I noticed the Kenya tree starting to droop and look sick then the pulsating zinnia started to look wilty and not pulsating. My protein skimmer was generating lots of junk more than usual.
Well the only thing that changed is the UV light in the filter so I turned it off. After 48 hrs all back to normal so i say yes the UV seems to kill off too much beneficial bacteria and it should not be used.

I've used one before with no ill effects. I think there's a lot of pros and cons to UV sterilizers and what works for one tank won't necessarily work for another. Do your research on them and make up your own mind. I wouldn't personally spend a large chunk of money on one but I can see their uses.

Make sure you get the right flow rate for your tank. One that has too low of a flow rate for your tank volume won't do much good. This is a common mistake and is another reason why so many people aren't keen on them and think they don't work.