I've had two FWB and in both cases, somebody got hurt. The first one was an ex-gf who I broke up with after she cheated on me. She wanted to get back together and was using sex as a lure. I knew that, and enjoyed the sex while it lasted. She finally realized it wasn't going to happen and broke it off. She seemed pretty hurt.
A few years later, I almost got married to a woman, but she wanted kids right away (the old biological clock) and I didn't (I was several years younger than she). We started hooking up again after about a year; this time, I was hoping we could get back together and she was just in it for the sex/companionship. She broke it off when she met the guy who's now her husband and the father of her kid. I knew the end was inevitable, but it still hurt.
I'm not saying it will ALWAYS hurt somebody, just that that's what happened in my FWB experiences. If it's just about sex and friendship to both parties, I think it could be awesome.