yeah, lol, maybe the discantage is that he needs to keep very concentrated or needs high level chakra, and if he go back to the past, he can't kill them. he has to fight with them, but in the past, is obviously that is someone's samrt, will utilise that when the enimies are sleeping or having a son, lol. maybe he can use the techinque, but for certains periods of time like 2 hours ago, or something, and then, he can't utilise the techiquie for 2 hours. lol, i'm inventing techniques xD