I think he's simultaneously over rated and under rated. I don't think he's as good as world class as his stats would suggest. However he's clearly still insanely talented and you don't get the stats he does without being a great player.
We've seen for England that he's not of the level where he can just make shit happen out of nothing in a non-cohesive team (whereas we saw Sterling being somewhat better on that front), but we've also seen that for Dortmund he's absolutely mint.
If he signed for United we'd see those inconsistencies play out and it'd ultimately serve them well. There'd be a few games where there up against a parked bus and frankly Rashford and Greenwood's abilities to whop it and Martial's brilliant dribbling might be more useful than Sancho's skills. But anytime they try and counter attack Sancho will probably put two guys on their arse, play a cracking pass and then follow it up himself with the finish.