I've never had a problem in the time that I've been using both. Again, keep your parameters in check. The nice thing about PU (not a derrogatory comment) is that it allows the calcium (read the bottle), strontium and magnesium to settle in situ. I personally have never had a crash or hint of a problem while using it. My biggest reason for using it is the diversity of color in the coralline. Maybe doe to different types of live rock, heck I dunno, but never a complaint from me or my friends.
Good luck and keep us posted!!!