I agree with Mgran, that fwb is very hard to carry off.
I think it is due to the fact that sex is more than just fun and games. It's one way we humans build connection. In a fwb situation, you either develop feelings for one another (hello complications) or just drift apart. The reason we seek out fwb is that one or more of the parties to the transaction isn't available for more commited possibilities. Good sex (and there's no point in having un-good sex) leads invariably to intimacy which then naturally seems to lead to commitment. How hard is it to feel intimacy with someone who (for whatever reason) cannot commit to you? Pretty risky, if you ask me!