Been in a few romantic relationships that deteriorated into FWB... wasn't exactly happy about it. Even been with a few friends that started out as FWB and turned into a romantic relationship. So I've never personally experienced FWB being even remotely stable... seems more like a transition or gateway... either to a romantic relationship or out of one and headed toward a break up.
It really doesn't work with women's natures... and the longer a guy stays with any female... the greater the chance is that he'll become possessive of her (may never actually 'love' her) or lose interest completely.
FWB is for the indecisive... sooner or later you'll have to make a decision --- start a relationship, end a relationship, or cheat.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen