Nothing in my experience - I used one for quite a while and had no problems with it. Some people complain about noise which never presented itself in my case, and a few people have reported getting cracks in the plastic housing that is used in this pump. If this happens you can always return it under warranty (If you're using a pump relay you can save your warranty, and get around cutting the pump cord by simply butchering an old extension cord, and cutting it to attach to the relay instead).
It's smaller than an Eheim, is cheaper, and looks nicer too (black plastic). So long as you don't mount it right on the floor of the case, which will cause a lot of noise through vibration, it shouldn't make any noise noticeable over a typical 30 dba fan. I actually forgot to turn mine off a couple of times by accident, and didn't hear it running overnight.