I think you have to look at every situation individually...
Louis Nix was not a 5 star, ge was a top 100 4 star, but is irrelavent to the 5 star conversation.
Stephon Tuitt was a star who lived up to his ranking. He was out of shape as a junior because he had two surgeries to fix a sports hernia that kept him from running and lifting in the off season. The fact that he left early and was still a 2nd round pick speaks volumes about his ability. His sophomore season was one for the ages.
Ishaq Williams was a certified bust. There is no defending that one!
Quarterbacks like Crist and Kiel are completely understandable. 5 star quarterbacks bust all the time at the nation's top programs. It's such a unique position, it has the biggest jump between high school and college, and college to the NFL of all the positions, and usually it's a fierce battle between multiple highly ranked candidates for the one available job. Lots of guys have 5 star ability, but they never grasp the game mentally, or the speed of the game proves to be more than they can handle, mentally. Also, it is probably the position (along with offensive line) most influenced by the quality of coaching you receive. For a lot of guys, their development hinges on them playing under a coach that can teach them how to turn their 5 star talent into production and success... The fact that those guys were busts comes with a huge asterisk for me because of the position they play and the enormous failure rate among quarterbacks, especially top flight schools
Greg Bryant, Max Redfield and Aaron Lynch will always fall into the "we'll never know" category because of off field issues. If I had to guess, Redfield would have started as a senior, never lived up to his billing, but certainly would have been better than the two freshmen Kelly started back there during the 4-8 season. He would have been a 3 year starter, but always seen as a semi bust. Greg Bryant would have been a really solid back in rotation with Tarean Folston and later, CJ Prosise. Probably never an Alabama type 5 star back, but a quality contributor. In Aaron Lynch's case he flashed stardom. He could have been a dominant DE and based on how good he was as a freshman, I think they hit the nail on the head regarding his ability, and as I previously pointed out, they rank talent, not character.
Again, it's one thing for me to throw a number out there, but looking at individual cases really provides context behind the various forms of "busts".