Anything other than the Premier League is considered a weak league these days. And although it is true that the Premier League is the best, you can not completely write off achievements in La Liga, Bundesliga, Serie A, Ligue 1, Primeira Liga or Eredivisie.
Sancho had 94 goals / assists in 122 games across 3 years, starting from when he was 18 at Dortmund. He was also playing well regularly in the Champions League from a young age. One bad season at Man United, a team that has been struggling, does not make him a bad player and he will improve a lot this season under Ten Hag.
For comparison, Richarlison has had 55 goals / assists in 135 games across 4 years, and he's now 25. Again, it is not bad but you could not compare it with the Sancho deal as a yardstick.