they can become an eyesore if they get out of hand and can block light to corals, so I would say definately not good. I have them in my tank but keep them in check with a sixline wrasse.
ive heard good things about flatworm exit. Thats probably your best bet instead of buying something you dont want. Or a nudi taht wont survive once they are gone, or will probably get sucked into a powerhead beforehand anyways.