What I have said is that during the Dubose days, when UAL fans were wondering what was happening to them, one of their members at one of their message boards posted a study on the relative success of DC and OC who had become head coaches. This poster found that OCs promoted to HC had in the last maybe decade had much more success than OCs. This was a study so of course some DC had had success as HCs and some OCs had floundered as HCs.
This finding was a bit surprising to the people on that board because they fell for the old coaching bromide defense wins championships. I was somewhat surprised, but since I had learned the lessons Spurrier taught us, I knew that offense was at least as important as defense. Now that study is somewhat old now, but until I see another, I will be more skeptical of a DC new head coach than an OC new head coach.
I also have come to subscribe to the old coaching bromide turned around. Rather than defense wins championships and offense sells tickets, I think it is offense wins championships, but defense keeps games close and saving for a while the jobs of mediocre coaches who stress defense.