I think it can be a little of both

I think if a coach tinkers with a player's throwing mechanics, it can really screw him up. If a new coach doesn't start to fix those tinkerings until the player's senior year, it's pretty tough for the kid to overcome that and perform well enough to play in the NFL.

That said, I don't know why people get so bent out of shape over Jurkovec specifically. Book has seemingly outperformed every quarterback in Jurkovec's class except Trevor Lawrence and Justin Fields (unless I'm missing somebody who was ranked pretty low and ended up performing very well.)

The biggest issue I have with Kelly is that his system sucks and I don't think it gets the best out of the quarterbacks (or much of our talent) in general. But if we're being intellectually honest and are going to rip him for not making enough out of Jurkovec, doesn't he also deserve credit for Book's success?

(IMO - the guy who should really be pissed at Kelly is Dayne Crist. Crist was ranked behind Blaine Gabbert (Career NFL earnings $22.2M) and ahead of Mike Glennon ($28M) and Andrew Luck ($97M).)