Not only you and probably most people doubt Ten Hag because he had a failure at Manchester United but we need to trust him and hope he can do much better. Kompany also did not prove to be a great coach when he came to Bayern Munich but he was able to give his best slowly. What made Kompany who he is now is because he has the support and quality players in the team. This is what Leverkusen's management needs to do so that Ten Hag can also give his best for the club's success. If he was able to achieve success as Ajax coach why not give him the chance to achieve success at Leverkusen.
The Dutch League is not the same as the Bundesliga and you can see an example of this in Amorim who was not immediately successful in the English League with Manchester United but in the Portuguese League Amorim was really not bad in coaching the Sporting CP team. So it's not just about past success that must continue to be expressed by some people even though it has become a fact recorded in history, but it is more about modern tactics that must be possessed by coaches and there should be no bigger egos that make players uncomfortable with the coaching style of a coach.