Lots of kinds of bad head coaches. Some are bad because they can't lead at the most critical times, they might delegate to crappy coordinators who are his friends, and can't understand why things are failing. Some are bad because they can't adapt. Some because they're past their prime. Some because they have no imagination and coach by rote. Some because they're fine at a lower level but have been promoted or hired to a level beyond their competence (The Peter Principle).
When I see Scott Frost's record, his career record, I see a bit of that Peter Principle; he was fine and very good as an OC, etc. And as a head coach in a weaker league.
I think about Alabama. Suppose the Huskers had every single starter that Alabama has. Would Scott Frost be able to produce, game after game, season after season, what Nick Saban produces? If so, he's just like Nick Saban but hasn't got the right players. If not, he's The Peter Principle personified; he's been handed a program in a league that he's not capable of helming to relevancy, let alone championships.
Maybe time will tell. I do have trouble seeing him as a great or even very good coach in the Big Ten, no matter the players brought in.