Sancho isnt a winger, he's a wide playmaker. The problem for him is that there are a few "undroppable" players who no matter how bad they perform wont be dropped: rashford, bruno and anthony.
Personally i think his best position would be at no10. But he won't get a run in that position because of bruno. Even if he did, rashford and antony are so one dimensional cutting into their respective half spaces that they lack the footballing intelligence to play with Sancho the way he'd want to pull the strings.
So he has to play one of the wide positions. However he needs players around him to be on same wavelength, to play the 1-2's and passing triangles. Unfortuntely for him, rashford is a shoe-in on the left. Which leaves the right wing. Hes technically better than antony, but doesnt do the off the ball work that antony does. Also we dont have a RB of sufficient quality going forwards to link with Sancho and play those inticate 1-2s etc, that he prefers.
We are back to the situation where we have a imbalanced squad, with players of different profiles who suit different systems and the zum of their parts isnt sufficient to make up for the deficiences in our gameplan.