GGPO (the fighting game rollback implementation) simply predicts that each player's inputs on frame N+1 is the same as frame N. This works well enough for fighting games but may not generalize, especially in games with precise analog inputs. Also, there's a hard requirement that in order to roll back 3 frames the engine has to be able to simulate re-running those 3 within the timespan of one frame, which is sometimes an issue when trying to patch this onto a finished game.