I understand your point mate, Ajax is not as big as Manchester United which might the reason why he could succeed in Ajax and couldn't in United. Some coaches cannot face the pressure when coaching a big club because they will be worried and confused due to critics from the fans. Those coach are known as average coaches. A professional coach will succeed in a big club despite whatever challenges that they are passing through. Look at Xabi Alonso who brought Leverkusen out from the darkness.
I think it is not about how big or small a club is, AJAX works in a completely different way, they have a very big and very good youth academy. They have always developed a lot of talented players and Ten Hag had access to them without transfer costs. And the Dutch league is not that competitive so it was definitely easier for him there.
At Manchester things were different but on the other hand he had a very good transfer budget and at Manchester United he tried to do the same thing as at AJAX. He picked young players and tried to focus on pace and attack. But it feels like he completely forgot about the defense and that was the weakest part of Manchester United and honestly still is.