People keep on saying he is too talented to waste his career. Well is he that good? Even Kagawa and Mkhi produced great numbers at Dortmund but they clearly looked out of depth for us. Maybe Sancho is similar.
I don't think the Kagawa or Mkhitaryan comparisons hold up to scrutiny. Kagawa was a very neat player, performing well in a specific role in Klopp's fantastic title winning side. He just wasn''t good enough to get past Rooney, or versatile enough for another position. On his return to Dortmund he also didn't quite manage to get back to his previous level, bar a few good patches of form.
Mkhi was/is an extremely talented player, but just very inconsistent except one brilliant season with Dortmund. I think his whole career shows that he always had the potential/talent, but for whatever reason only shows it in glimpses, whether in Manchester, Arsenal or Italy now. Hardly unusual to see such a talented player not capable of living up to it consistently, happens with many players.
I think Sancho's time in Germany was far more impressive, and consistent over a longer period of time. Personally I think his current problems start with lack of confidence after the world cup, joining a dysfunctional team and what seems to have been a very toxic dressing room at the time. That kind of environment for a young player, and being low in whatever hierarchy this team has, is ultimately the main reason he's struggling in my opinion. For better or worse, In Dortmund there always seemed to have been a fairly flat hierarchy with leader figures like Reus or Hummels happy to let the young players shine. And Sancho was clearly one of the players everyone tried to play to to make things happen. That doesn't seem to be the case at United, because he's obviously been disappointing so far, there are bigger stars in the team and his confidence is shot. Not sure he can turn that around by himself, or perhaps it'll solve itself once the team has more positive momentum. But I wouldn't discount his performance in Germany just because it's not working in Manchester right now.