McDavid's true value is that he is the best player in NHL history at creating odd-man chances off the rush: creating a breakaway by blowing past the D, beating his man to create a 2-on-1, 3-on-2 etc.
The issue in the playoffs in twofold: first, players play harder, and will give more effort backchecking, so it makes it harder to create those chances just due to skating. Also, teams recognize that he is far more dangerous off the rush, so they play a safe game in the OZ, and layer the NZ so he always has a guy in front of him, which pushes McDavid to the walls unless he makes an amazing move. And it's proven to be successful.