Additionally - if you look at some. of the feedback there seem to be some issues reported about a black substance when wiping inside the unit.. However, its unclear whether this is a problem for the tank - or whether its similar to the 'gunk' (black/brown) that builds up in most PVC used in tanks over time....
PS - as to the flow - generally it's thought that depends on the size of the tank, the size of the UV and the goal of the UV. Slower flow is generally best for disease. But - IMHO - UV for disease prevention may be somewhat over-rated UNLESS proper maintenance is done. Also IMHO - one can get the same benefits without using a relatively expensive (if best quality) - device that as already seen is difficult to set completely properly with regards to dose. There is also a debate about using it constantly (due to killing of microorganisms in the water that may benefit corals) - and people who recommend using it only 6-12 hours per day - which will also diminish its disease fighting (and algae fighting) abilitiy. Having said all of that (sorry it's long). Many people use them and defend their use. I have done both - and saw no difference.