I think you're wrong, I don't think is about the manager Vincent Kompany that's making the team not to do well in the champions league and also is not about lack of experience by the coach. If is because of lack of experience and the coach himself then why was Thomas Tuchel not winning every game of the champions league and why didn't he win the UCL title since he has the experience and has won it before? This isn't the problem, the issue can't be pointed out, to me I'd say that other clubs have stepped up their game and they're now used to the playing pattern of the Barbarians, is not about the experience of the coach.
No issue in football can't be pointed out. All you have to do is look close enough. Most times it's not just a single issue. Different issues can be wrong and a club can suffer for it.
Sometimes, it could be just a simple dip in form that is caused by different factors.
For Bayern one of their issue is that they're not clinical enough and their defence is not that good. After the matches today I looked at some stats on the UCL website and I saw that Bayern Munich has created 81 attempts, but scored 11 goals. They should be scoring way more than that. Harry Kane is the only clinical attacker they have. Gnabry, Sane, Coman and Musiala are not that clinical.
Another problem is their defence. If you want to be missing chances, you have to make sure you don't concede. We saw how the defence struggled against Barcelona. they need to work on both these things and they might get better performance.
I agree with you on the hidden issues clubs have and if you not observant it could be a minor issue but that aside, Bayern Munich have been one of the best in terms of champions league football and you won't tell me that these are not the same guys that have been dominating Europe for quite sometime now, although we have few players who haven't won the UCL title with the club but they are still that Bayern Munich unless they have to make some changes like bring in new players. If it can be done players like Sane and Gnabry ain't suppose to be in that team, they have lost that sharpness to pass through defenders like they used to do.
I'd say this is the season where their defense isn't good enough and forget about stat, they always create chances and score less and that's Bayern for you. I won't blame their defense but the whole team, they seem tired like they have been exhausted and they need rest but they'd their problem to recover from if not they won't go far in this champions league.