Disagree
RGIII only success as NFL QB came under Shanahan. He was one who drafted and started development of Cousins
He developed Jay Cutler into PB QB (Before Bears setback all progress)
He turned Jake Plummer's career around
He got production out of Brian Griese
He was major reason for Young/Elway success
To say that QB Development is not something Shanahan would succeed at is laughable
If RG3's only success came under Shanahan then his descent to failure came because of Shanahan as well. Either take the good with the bad or call it a wash but in either instance, it's not a positive for Shanahan. Cousins has a whopping four starts in his two seasons under Shanahan. That is hardly enough to take credit for any level of development for him.
Cutler's two seasons under Shanahan in Denver is not particularly impressive in the grand scheme of things. His one truly great year in Denver still ended up in Shanahan getting fired because they couldn't lead them to the playoffs.
Turning Jake Plummer's career around? He was the same inconsistent QB he was before just on a better team which inflated his numbers slightly and made him look better than he was. I'd call that a stretch.
Griese had one good year.
Shanahan was never a big reason for the success of Steve Young. And Elway was already a great QB before Shanahan showed up.
So at best you can say that Shanahan is inconsistent with developing a QB but his coaching and record pretty much proves that he's good with a good team but he doesn't have much of a track record starting from scratch with teams. Plus, I just don't think he knows how to handle players these days. After Elway retired, his teams went 115-109 and won one playoff game in six tries and missing the playoffs nine of the fourteen post-Elway years.