UV only clears water if you have an algal or bacterial bloom, it does not filter.
It also does not effect beneficial bacteria because they are not free floating, just as it does not effect algae growth on glass or substrate.
As stated above it will only kill parasites, if the parasite is exposed to the UV long enough, ie slow enough flow thru the UV unit.
If you do not regularly clean out the tube that protects the UV bulb, it becomes ineffective due to buildup of minerals and debris.
UV bulbs also lose effectiveness over time, and should be replaced just like fluorescent bulbs.
I would not say 200gal per hour is high, but may only kill very sensitive organisms.
Not trying to be smug, but most of my tanks have pumps that are at least 1200gph, unless they hold very sluggish water fish like bettas.