Who are the great quarterbacks that have struggled at coaching?

My best guess is that the spoils of being a great quarterback are so great, and the retirement job offers are so lucrative that they simply don't want to take the steps required to learn to coach.

For example a pretty good QB turned great coach like Jim Harbaugh took a job as an assistant coach Western Kentucky after he retired, and subsequently moved up the coaching ladder. I struggle to see a great QB taking a road like that when they have so many other options.