As far as the X's and O's are concerned, there is very little difference between college and the NFL. As a matter of fact, I would say the NFL is harder to scheme against. Would recruiting be a concern? Yes. But thats why you hire a great recruiting coordinator and more so a great staff.
There are major differences. One of those major differences is that you're faced with more limitations in college. If you craft an extremely complex offense, chances are that several players won't be able to grasp it. Those players, remember, have all kinds of other duties (e.g. classes) and aren't full-time football players making big money. Success in college football often has more to do with talent development than X's and O's. I have no doubt that Gruden could improve play-calling in Year 1, but I have significant doubt that Jarrett Guarantano could develop into an elite QB under Gruden, whereas, I know Dan Mullen will get the most out of JG's talent.
If you look at Jim Mora at UCLA, you see a similar pattern. He improved play-calling and game-planning almost immediately, but player development has fallen off. So he had a few good seasons early-on, but the program has declined overall under him.
If you really want to understand why Gruden could fail in the NFL, I think it's most instructive to read about Callahan's failure at Nebraska. Remember that Callahan was Gruden's OC at Oakland.
Doomed to Failure: The Bill Callahan ClusterFool
Callahan's NFL sized playbook simply overwhelmed players. Thousands of plays...all in search of perfection. Players only got a limited amount of time to digest each part of the playbook before the next lesson... there was no time to teach or correct things during practice because the coaching staff had already moved on to the next lesson. So each player muddled through as best they could. And boy, did it show on gameday. Sometimes running three or more different plays on each snap, as confusion reigned supreme.
The funny thing is that oh so often, the opponent was better prepared for what Nebraska was trying to do than Nebraska was. Or was that the sad thing. Or was it so pathetic it was funny?.
So the biggest takeaway here is that Gruden's great X's and O's in the NFL won't necessarily translate to the college game where he's going to deal with a much more limited playbook. Many coaches have said that Gruden's biggest weakness in the NFL was talent development; but that's one of the most important parts of the college game. I think Gruden would have more success than Callahan simply because Gruden's personality is better than Callahan's, but he also has some of the same weaknesses as Callahan with an overly complex offense and he's not good at talent development. Perhaps Gruden could find assistants that would make up for his weaknesses, but to me, I just can't view him as a top-tier candidate compared to coaches like Mullen, Frost, Fuente, or even Brent Venables.