Bayer Leverkusen is one of the strongest and most successful teams in the Bundesliga. The team's squad is well-established and has a structure inherited from Xabi. This wasn't the case at Manchester United, which also had a challenging Premier League campaign. Therefore, I believe he will be more comfortable and capable of greater success at Leverkusen. The German league is well-suited for this, but you won't find such comfort in the Premier League. Of course, I underestimate the Bundesliga—it's one of the best in the world—but the other league we're comparing it to is the Premier League. I think you get my point. 
The structure left by Xabi for Leverkusen is that in reality some of the important pillars of his squad have left so of course this is a big challenge for Ten Hag and besides Leverkusen did not buy many great players, they only spent a total of € 53 million on several young players and their quality is not enough to replace the players who have left, of course I can't immediately criticize when the league hasn't started but with what Leverkusen and Ten Hag have built now I feel that Ten Hag will have a hard time competing even though this is not the Premier League.