I just watched the game again and man he really struggles with his reads.
Boren's fumble was flat out Miller's fault. The end crashes right into Boren as he is getting the handoff from Miller. Miller without a doubt should have kept that ball.
It appears to me that he is deciding what to do with the ball before the snap on most option plays.
Miller is so good at scrambling that it covers up for a lot of errors, but if he can develop his option reads the offense could really take off.