He hasn't been good, no. Even in his ok games. He looks sluggish and lost. Obviously these are not the kind of performances that made everyone rave about him, so don't worry on that front - it has nothing to do with some intrinsic league difference, even if moving to a new league and team of course matters. The Bundesliga isn't actually
that different or
that much poorer that this kind of showing would somehow be considered amazing
Utd need to have patience with him while sorting out their team, style, management, he's an exceptional talent and while there's never a guarantee that even great talents can't flop, I'm sure he will come good over time and you will have loads of amazing moments provided by him. For what it's worth, us Bayern fans had to have a massive amount of patience and optimism with Sané (PL YPOTY after all..), who was downright horrific for a full year (much worse than what you're seeing from Sancho) before improving and now slowly coming good.