Jerry Jones was right: no coach has ever had success when changing teams - no one has ever won a championship in one city, and then gone on to coach a championship team in another.
Some have come close, but no one has ever done it. Not Shula, Parcells, JJ, or anyone else.
So when you feel the desire to bring in a Jon Gruden or a Bill Cowher because "they've had a history of winning" remember that history has a way of repeating itself in the NFL, and they are unlikely to have success here or anywhere.
Plus this: when you hire an uber coach, you cede power to him. He buys the groceries and cooks the meal. He's the GM and coach, and therefore he has control. The current NFL makes that really hard, and there's no one there to help balance the equation. Its all about ego, and its hard to have success that way.