I agree with Mc, the evaluation of Defenders and Midfielders is a lot more dependent on the overall quality of the teams they play in than forwards and goalkeepers, in my opinion.
Forwards and Goalies are the two most individual positions on a field. Forwards are expected to contribute to the team’s overall play but they still get evaluated mostly on their goals. So a forward can play in a poor team and be rated highly if he scores lots of goals.
Likewise goalies. It is actually easier to look good on a bad team because you get plenty work to display to talent evaluators.
Midfielders, and to a lesser extent defenders, are much more dependent on the players around them. Even if you have a standout midfielder, if every ball he receives is bobbling, off pace or misplaced and if every team-mate can’t trap a ball properly or run into space to give options, or if players don’t recognize and reward his good movement, his effectiveness is limited. I see plenty good players, touches men, who are anonymous on a bad team but surround them with good players and they start pulling strings and look like stars.
Defense involves similar teamwork, particular communication and covering, but not as much as midfield and so it is no surprise that our defenders get more opportunities abroad as well. And when they get the chance they have an easier time adapting. You could also evaluate defending skills in isolation more easily as tackling, heading, speed, marking etc.
Ultimately the position where our players have the hardest time adapting is centre midfield and a lot of this has to do with the general development of our players. The learining curve is just steeper for our midfielders and clubs don't have the time.