He was and is a film junkie, who was and is great at breaking down and explaining plays, and commanded an institutional knowledge of football schemes that was probably on par with Brady's.
But, some of the problems I saw with him as a HC, were some of the same problem I saw with him as a player: an inability to apply this institutional knowledge at game speed, to react instinctively instead of thinking. As a player, he had ZERO instinct and was almost always half a second behind on every play. You could see him questioning himself, doubting, and thinking too much.
And as a HC, he just couldn't keep up with how fast coaching adjustments are made within the game. I kind of tuned out on the season after seeing him get his ass beat badly by McVay.
I'm sure that if he could play chess without a timer, he would win handily. But when the clock is ticking down, right there on the table, he chokes.