One of the reasons for Ten Hag's failure with Manchester United was a lot of star players at Manchester United's squad. The bonding between the players was weak. It was also very difficult to control a bunch of star players. ten Hag had not enough experienced to control the squad with so many star players.
But Leverkusen's condition is different. The players here are very diligent. They want to do well. The relationship between the players is also very good. If Ten Hag gets a chance to work independently, and is able to create a stable match strategy, I think we will see better performance from Leverkusen next season.
They didn't really had any "star" players, look at the squad, who is a star among them? I would say Bruno is clearly a star, but aside from him, there isn't really one, all the others are expendable players. Maguire gets some chaps from everyone, even I did it, and wished him gone, but he showed up day after day so I respect that grind at least.
And in the end, ETH just made bad transfers, he could be a good manager or a bad manager and you can discuss that, but he was a terrible scout that is for sure, the players he wanted were mostly terrible ones and he made mistakes of playing them badly. If the players were hired without his knowledge, then I would understand, but reports dictate he wanted almost all of them himself.