This is the catch for me.
Son works best coming off the left, IMO. It suits him, allows him to find space and to run at people.
He is very effective as a CF replacing Kane as well, which makes him an incredibly valuable squad player.
Kane works best up front, furthest forward and banging them in. He is VERY capable when dropping deeper, and can be used to good effect doing so in game, but to effectively play him there neuters him.
Which is why I cannot understand Pochs persistence with a Son/Kane front two. Kane is sacrificed to Sons benefit, but neither are at their best in that set up.