Burnley is mediocre team but still, Vincent Kompany manage to guide then to become one of the fiercest mediocre EPL team, promoted them to English Premier League, but somehow they got demoted because they failed to meet the requirements. Perhaps Vincent Kompany found favour in the eyes of Bundesliga giant, he have more work to exercise in his new club, definitely going to be tough for him in the first season because there's more handling responsibilities to take of. The players in Bayern Munich are no mediocre players but experienced players that knows every corner to win games.
If couches are being appointed based on their previous performance in their previous club, I bet you majority of them will remain jobless till the rest of their life. Let look at Wayne Rooney couching career and see how he relegated the clubs he has managed and some clubs are still after him looking for his service. Likewise, Graham potter was doing well in his former club Brighton which made Chelsea to go after him but he could not deliver well at Chelsea during his stays until he was fired. So, I think it has less to do with the previous performance and can not determine the success of the present. Vincent kompany is not too bad and considering the squad that Munich has, he might probably perform well if the team is ready to work along with him.