Without even bothering with any sort of forensic analysis, I can confidently say that it is of course, nonsense, that a player is more likely to fail because of the country he played football in previously. Just on the basis that footballers are all different human beings and ultimately, if a player has the tools to succeed, he will do. And if he has them, he will have them regardless of whether he lived in Frankfurt for 2.5 years or not.
Players have succeeded in the PL having come from Norway, Australia, Holland, Russia, USA, Scotland and almost every other country. Regularly. There’s nothing about Germany in and of itself, scouts need to be thorough but Germany is a good enough level of football to assess a player’s qualities.