I don't think there's a version of FF that demands no failures. The example you were responding to seems to me to be pretty solid. Honestly? Where FF gets less easy to apply is when we're dealing with very atomic little actions that don't carry any of the weight of the context with them. "You fail to pick the lock" as an isolated atom of fiction doesn't really leave much to be worked with, and if you absolutely insist that every instant of play be fully diegetic within itself, so that you can't bring in concerns from the larger context, then how would you do FF? You can't! I mean, you CAN, but then the only option you have is to say "OK, you picked the lock, but someone heard you." Like, you kind of create your own issues, which is fine, do what you want, but don't attribute the limitations of those techniques to other concepts, like FF. FF works great!