Winger is its own world. It is about knowing where to be in space, knowing when to crash the net and, more than anything, going into corners, banging forwards, and cycling pucks.
As someone already said, this works in Engo's favor. He can ask coaches where to be in space, he knows how to crash a net and with his big body, get into position, and he definitely can go into a corner and bang it with the defense.
From what I see, Letang is very poor with the puck but yet likes to have it as much as possible. Wingers barely ever touch a puck for more than a second as they are either cycling or working it around or to the center. Letang would be lousy in the slot as he is too small. He is not a corner mucker. I just think he would be terrible on wing. Center would be better for him giving his skating and range which are his strengths but the simple truth is, if coaches would just force him to play his position, he could be a world class d-man.