Kagawa and Mkhitaryan weren't cut out for the level, the clues were there before we signed them. It's a psychological issue rather than a question of talent. A tougher field makes players without that elite mentality shrink. For Sancho it's early days but his talent level is higher than the other two Dortmund players but he is more on the introverted/shy side so will take time to adapt and gain confidence. Bellingham is the total opposite of those players. As Hummel's said he's the most mature 18 year old he's ever met and is already a team leader for Dortmund. His confidence and competitive nature is off the charts. I'd be surprised if he came to England and fell off because he's completely fearless.
Whilst the Bundesliga is a lower level than the PL the idea that there's something wrong with Dortmund players or players from that league is nonsensical. Players can come from any league and adapt if they have the ability and mentality to do so. We've signed players from them with only the ability only in the past. You should look at Bellingham in isolation from those other players as he's a totally different character to all of them.
The toughest league I've ever seen was the Serie A of the late 80s and mid 90s. It was common even for smaller Serie A to have WC players such as Dunga, Batistuta, Careca, Maradona, Klinsmann and co. The situation grew so ridiculous that at one point small teams had top players such as Taffarel and Hagi could end up playing with small clubs. Under such circumstances it wasn't uncommon for top players to go there and fail spectacularly. I am referring to talent like Denis Bergkamp and Ian Rush who failed in the Serie A big time. The EPL in particular was frowned a lot at the time as it was seen as a league that either produced vastly overrated players (ex Walker, Wilkins, Blissett) or they lacked the professionalism needed to succeed (ex Gazza). Success stories at the time were rare (ex Platt) and the Serie A would look down at the EPL up until Paul Ince showed up at Inter.
The EPL of today is not quite at that level yet, however it reminds me alot of the Serie A of the time. Similarly to that Serie A there is no easy games, talent is fount everywhere not just at top 6 clubs, each point counts and defenses tend to be quite difficult to crack. Similarly players who might be doing exceptionally well elsewhere might fail at this level.
I think that the Bundesliga is nowhere near to the EPL. Bayern are a top club but the rest would struggle playing in the EPL week in week out. I would have no issue buying players from Dortmund if its worth the punt. Unfortunately we all know that it ain't the case with them.