Can't help but think that Di Maria is the level Dembele would reach without injuries. I've been extremely frustrated seeing Di Maria play, taking shots from impossible angles, drilling his head down and ignoring better options etc.
Footballing IQ is relatively low, but he has improved a lot and at times does amazing things as he is incredibly talented. Dembele is basically the same player, difference is the Frenchman's 'amazing things' appear more rarely.
I think consistency is an issue for both, with Dembele being way behind mostly due to lack of playing time.