Pretty much echoes my thoughts. If we had Sancho, he would be our best dribbler by far. He's averaging 3 dribbles a game, while Rashford averages 2 a game. Sancho's tricky feet is something Martial has, but Sancho has managed to turn it into a much more lethal dribbling game. Therefore, in packed boxes, Sancho would be our best weapon. It is for this reason that Sancho would also be our best creative player to get assists. Technically Pogba would be, but he's in a deeper position compared to one of the front 3.
Rashford, Martial, and Greenwood are all forwards/strikers. Sancho is more of a winger/forward. It's a clear deficiency in our side.
After Sancho, we're missing a dominant midfielder who is a constant presence in offering himself to receive a pass, to distribute the ball, to and to resist a press. We're essentially missing a Scholes, Modric, or Xavi. Doesn't have to be as good as those players, but we're lacking that type of midfielder.
Two glaring deficiencies to make us practically complete, but not the finished article in terms of quality.
Don't think he's great at it. Think that's something that develops at first team level because there are more chances to head at first team level. Possibly due to the passing not being up to par, or because strikers focus on other types of goal scoring at a young age.