He won a Superbowl in his first year in TB with a roster that was mostly intact when he got there. His overall NFL HC record is 95-81 or .540. That's not awful for the NFL, but it is not exactly gang busters. Belichick is .673 overall and .739 in New England. Andy Reid is .608 overall, both having coached far more games than Gruden. Mike Tomlin is .644, having coached roughly the same number of games. In fact, there are 18 active HCs in the NFL with a higher career winning percentage than Gruden.
He has never ran a college program. He hasn't coached AT ALL at the collegiate level in 28 years. He's just going to walk in and start winning? There's more to it than X's and O's. He has to recruit. He has to run the program. He has to deal with the NCAA rules and regulations, etc.
I am not saying he can't be successful, but, IMO, he's at least as big of a risk as many of these "up and comers" that so many seem to be afraid Tennessee is going to hire.