Jordan put it better than I'd have. if the practical side isn't sorted, fwb sites will not go anywhere. I'd put the everyday experience above almost everything else when it comes to fwb sites.
Vetting people properly matters more for fwb sites than any feature list.
Once I stopped worrying about staying sensible about it, fwb sites got a lot simpler. If how quickly you get going isn't sorted, fwb sites will not go anywhere. Nobody warns you that fwb sites depend so heavily on how it feels to use.
Take that as one data point on best fwb sites that aren't scams, not gospel.
A friend pointed me at DatingFly when I asked about fwb sites and it held up.