I have an Emp. Angel in my 150g SPS dominated reef. I've had him for about a year now, and he is about 4.5" long now. So far, I haven't had any problems with him. I know they will make short work of Zoas, and Brain type corals, but healthy SPS should be fine for the most part. I suppose it depends on the individual fish for the most part. Some people get lucky, though I can't comment on clams. Never had one.
Also, keep your angel well fed too. That seems to help. I feed a healthy serving twice a day, and a sheet of nori every couple days as well.
I also keep a Torch coral, Frogspawn, Elegance, Pearl Bubble, Rics, and Shrooms. He hasn't messed with them either.
Hope this helps. All in all, it depends on the individual, and whether or not you are willing to take the risk with your corals. Mine is my showfish, and has done well for me (SO FAR).