I think the answer has to do more with the degree of abilities, both in comprehension and the natural physical endowments of the pitcher, plus the communication skills and the time devoted to the coaching process, the facilities used and certain intangibles.
I've had considerable problems bringing some pitchers around to enhance their craft only because of the lack of facilities, investments $, and certain quality of life issues.
On the other hand, some require little in the way of hip-n-shoulder disciplines, and they do very well for the level that their at. Now I'm not talking amateur ball here.
Besides, this focus on hip-n-shoulder separation I'm sure is important, very important in fact to generate torque and drive, but, so many other things come into the picture that the topic of conversation - I think, should be a bit more dynamic. Dynamic in terms of conditioning for whatever season the pitcher is in, age and muscle maturity considerations, diet and nutrition standards, sleep maintenance and so forth. I also think that from a coaching standpoint, coaching amateurs - especially youth, so much is out of the control of a coach and up for grabs depending on who and what the pitcher is - age, maturity, etc.
I know this doesn't get to the heart of your post, but it's the best I could do at the moment.