Let's see if a club like Leverkusen can still be a competitor to Bayern Munich after Leverkusen is in the hands of Erik Ten Hag. No one knows for sure until everything is proven, and I will not praise Erik Ten Hag too much for a career that he has not been able to prove at Leverkusen. The Bundesliga is a different story and, but I am quite happy that he is back as a coach and gets a famous club.
This would be the biggest test of Erik Ten Hags life. He was ok at Ajax, everyone liked him and saw him as decent, but when he went to United he was bad. Now that he is at Leverkusen, it is going to be questioned "was united bad because of ETH, or was ETH looking bad because of United?". If he does great at Leverkusen and be competitive again, even if he doesn't win but finish second, then it is going to mean he is doing great, and he didn't suck at United but United just sucks.
But if he does terrible, be outside of top four, then it would mean he will get fired yet again, and then he will have hard time finding any job at all. That is not going to be easy for him, he wouldn't really consider that as a bad thing, he may never manage a good team again.