No body believed PSG will win inter Milan and lift the champions League trophy and no body believed Chelsea will win PSG and lift the FIFA club world cup, so no one should judge Ten Hag now untill next season starts.
It's a common problem, there will always be people arguing to predict the future. It's also the same with me who often comments and predicts the future of players based on previous statistics. But I wouldn't say Ten Hag failed, I think the EPL is a different league so it's difficult for him to get the best achievements. Remember he is from the Netherlands and the league conditions are far below the EPL. Now Ten Hag is in Germany, maybe the league is above the Netherlands but below the EPL, so I think Ten Hag has a great opportunity to make new history continuing Xabi Alonso's good statistics.
I disagree with you, because Bayer Leverkusen doesn't have the resources Manchester United had back then. Manchester United signed whatever player Erik Ten Hag wanted. Despite this, Ten Hag wasn't successful. In my opinion, he's not a top-tier coach in terms of his technical career. He may have had a successful record in the Dutch league, but he can't demonstrate similar success elsewhere in Europe. We'll see what he can do in Germany this year, but their defeat against Flamengo's U-20 team in Brazil is unacceptable.