I would guess that it is going to be not so hard for Bayern to win, just because Galatasaray may have gotten some lucky positions and beaten a very terrible United doesn't mean that they will be capable of beating Bayern, don't get me wrong they could be second and go out of the group stages, but they are not beating Bayern, not even getting a tie. Kane and Sane and all the other players will make sure that they end up with a good group stage where they end up beating everyone, a 18 point group stages carry some of itself into the later stages because the team will have some amazing morale and belief in themselves to push through any difficult team to win.
I believe that last 16 will not be that hard for them, they are better than almost ever second team out there, but then quarter finals will be hard, it is going to end up having some sort of situation where other groups first places will be playing with each other mostly and Bayern could draw someone very strong there.