While the level in the PL is definitely higher (everything else would be a disaster looking at the money involved in both leagues) I think the difference in preferred playing styles might also be a factor. German teams seem to play higher lines than many English teams, so players who can beat those high lines by speed (like Timo Werner) or the ability to play through speedy players (like Sancho) is quite valuable in the BL. On the other hand players who are used to work with very little space in the PL are struggling to beat the distances they have to due to higher lines in the BL (look at Sadio Mane or Callum Hudson-Odoi having very little impact this season so far).
Haaland is exceptional because he has both speed and power, that's why he can beat every defense. This is of course an oversimplified view of the leagues, but I still think we should keep that in mind as a potential influence.
Back to Sancho I'm also convinced that there is a mental issue for him. He did put in a shift at Dortmund and he dragged them to a cup win, he was a leader for that team (along with Haaland, but Sancho was definitely more important to that run). We see nothing of that at United, instead we see a mangnified version of the Sancho who started slowly into seasons and got sanctioned for partying. Somehow it seems like a huge mismatch and after this is going on for the whole time he is there I do feel like it would be best for him (and United) if he rebooted his career with a fresh start somewhere else.