It depends on how you define success, because in baseball, sometimes success is dependent on luck. If a guy's stinging balls all over the place but they're getting caught, should he change his mechanics? Also, in jbj's case, I've heard some analysts/scout types say (granted, they're from here, so not your typical ESPN guys) explain long swings as "slider-type" bat speed (read that in scouting reports from the states too), as in, mechanics with unnecessary movements and inefficient bat paths that make it hard to catch up to big velocity, forcing the batter to cheat on fastballs, then be easily beat by breaking balls.