It's a little unfair to single out FWB relationships when in reality almost all relationships never work out. In a world of apparently endless choices and options (no matter how illusory most of these actually are) the fundamental tensions in a two-person relationship are likely to result in a breakup sooner or later, whether one is married, in a FWB setup, or simply casually bumping into semi-compatible folk. Emotional immaturity is often a major contributor, and this isn't something confined within one form of relationship. Indeed, most of our work relationships suffer from it as well, never mind our relationships with friends and family...