In short, it is far too qb centric, forcing the qb to make too many decisions/reads rather than just execute plays. Expecting an offense to be efficient/effective only when there is a "great" qb, is ridiculous since there are really about a handful of great qbs in any given year.
We have seen qb after qb fail to achieve the growth you typically see in qbs running their offenses in the 2nd years. In fact they seem to digress, rather than progress. Kelly has had talented-enough qbs ( for normal college offenses to be much better than his anyway) but none have ever really gotten better as their careers progressed. It would have been nice to see what Kizer could have become had he stayed one more year but even he knew the offense design was the problem not the players. So he he rolled the dice but any chance he had of becoming a better qb and learning a real offense was shot to shit when he had to start as an unprepared rookie on a truly bad team.
We shall see if Book excels -- and he certainly ought to since this will be his 3rd year running this offense and his fifth year in the program.
Qb isn't the only position hampered by the Kelly offense. Take a look at the offensive line which year after year after year under performs to the level of it's talent as witnessed by what our grads do in the pros.