McDaniels isn't a great OC, he coached the best QB in NFL history. A good OC would build a system that wasn't too complex for the most important position on the field. They would build one that suited that player. McDaniels thinks he's great and isn't. Gruden designed a system that fit Carr and Carr had his best years in the NFL as a result.
Your last point is actually really good. He isn't a good fit for a system that they won't alter to fit the players they have. That was literally the reason they had Gruden come in during camp, to help them fix that. Unfortunately, that died when he left the building and Pete went back to what he knows. Which IS the problem we've had with all of the QBs over the last two years. A refusal to adjust to the players we have, and coaching as though we have the players we want.