There's a narrative going around that, John Elway aside, Mike Shanahan has a poor track record of coaching successful QBs. The argument centers around the fact that Shanahan has not won a Super Bowl without John Elway as his QB. But the argument is incorrect.
It's true that Shanahan has not won a Super Bowl post-Elway, but this is because Denver has had poor defensees (*this* would be a riper area for a critique of Shanahan -- his defenses in Denver were often terrible). An honest look at the data suggests Shanahan's QBs do very well.