My take, is people think Bo didn't know how to coach defense, when in fact he is a defensive genius, he just doesn't know how to coach kids. Some confuse and/or don't know how to separate the two.
Listen to the PJ Smith interview last year, listen to Nate Gerry just today, the defense didn't communicate or listen to each other. In his scheme that relies heavily on communication, certain players were being selfish and not communicating, trying to be the hero and make the big play, instead of playing their role and allowing teammates to make the play. That falls on the players, but it also falls on Bo and his assistants not finding a way to get through to those mental midgets.
I think he is a smart man when it comes to football, but I'm glad he is gone. Being a sm mind doesn't equate to being a good teacher/coach and player developer. He worked out OK in the beginning because he had players that were sick of Callahan and those losing ways, that grasping onto his new, fresh ideas that showed results were worth trusting him. He then failed to grow that, let alone sustain that level of play. He lost trust in his players and as it appeared, control of a program.
Glad Bo's gone and glad we have Riley. There will be flaws we find in Riley and his staff, but there is plenty more upside to like and be optimistic about. Coming from a high school program that spent at least 15 years going to Oregon St football camps, knowing kids that walked on for him, our coach almost took a scouting job under Riley while with the Chargers, other coaches of mine - one being my friends dad - that met him and got to know him, there is a lot be happy about and not just off the field traits.