I generally think defensive coaches make better head coaches. Offensive coaches tend to only think about offense and how to score points. Defensive coaches have to take more of the game situations into account since they don't control the ball.
But if defensive coaches have a blind spot, it is for running QBs. They seem to love them. That's because in theory a running QB is impossible to stop and they know it. They can likely think of hundreds of plays in their career where they had a perfect defense called and a QB ran for a big play.
They just assume you can teach him to throw from the pocket. You can but the problem is it often takes years and by the time he becomes a great pocket passer, he's no longer a great runner. And you're fired.
Mike Vick is as talented as a QB could possibly be. One of the strongest arms of all time, great touch, and could have been a pro bowl RB or WR. All of that and he couldn't win consistently because he didn't put in the work. He blew off his coaches.
I see Manziel going down that road. He will make some highlight plays, be a good fantasy QB but lose games in real life.