No one is questioning Sane's ability, but I think there is concerns about his desire to get to the very top of his trade.
In my opinion he has even more ability than Sterling but no where near his desire.
I think one of his biggest negatrives are his one dimensional dribbling.He seems to always make the same moves over and over again and seems he can only go to one side.He is perfectly fit for counter attacking football,there he is absolutely lethal ,but playing against bus parking teams, you need a bit more variety on your moves,thats why sterling is better there, he is small ,has insane acceleration with the ball.