I'm not saying all Bundesliga players are crap. There's plenty of anectodal evidence without Fuchs to show that Bundesliga players can come good. I think Sancho will come good.
The issue is that during the first 6 months to a year or so they look seriously off the pace, which is strange because there is a reputation for the Bundesliga to be quite fast paced, end-to-end and similar to the PL. Watching Thiago, Havertz and Werner last year was a right laugh but they are better this year. Compared to players that come from the French/Italian/Portuguese/Spanish leagues, it feels that the settling in period for BL players is much longer. If we gave Kagawa or Mkhi a couple more years, they'd probably be alright and playing at the levels of Sane, Mertesacker, Fuchs et al. But man, that's a lot of time.
Yes, all new imported players take time to adapt to a new league, a new club, but it does feel that Bundesliga players take that little bit more time. There are, of course as always, some exceptions to this. As I mentioned before, KDB, Son and Auba hit it off like they have always been here. But for most others this isn't true.
I'd love to see a stat of this across several leagues but how does one measure 'average time taken to settle in a new PL club" over a large sample size? How does one even measure 'settled in'?