You may accuse Erik Ten Hag of failing to lift Epl, maybe that was the failure you mean. But generally, I don't think ETH failed in Manchester United where he spent 2 years with two trophies. He ended up qualifying the team for Europa using the FA cup in his last season. Bayern is a difficult nut to crack in the league and everyone understands their dominance. Leverkusen under Xabi Alonso was also great and full of passion but it would be unfair measuring ETH with their previous achievements knowing fully well that most of their players are no longer there. Maybe Erik Ten Hag would clean up his name later in the season because currently, there's absolutely no hope for the team and I don't see them as one of the title challengers.
That's not an accusation, but rather a fact of what happened previously during his time as Manchester United manager, and you can look back at Erik Ten Hag's time as Manchester United manager. If relying solely on trophies might be true, but his final season at Manchester United showed that Erik Ten Hag wasn't as good as expected, despite the support of several players he bought. There are reasons for this assessment: first, he looked like an incompetent coach during pre-season, and second, several key players from Leverkusen's previous squad have departed. In other words, it will be very difficult for Erik Ten Hak to make changes in his first season with Leverkusen, and there may be no hope of challenging Bayern Munich.