It will be hard to truly grade how bad of shape Frost left the program until we see what happens when the next coach comes in. There is a difference between who did the worst job and who left it in worse shape.
Callahan's biggest fault was removing our identity. He ignored many of the traditions and things that made NU great and did his own thing. He did do a pretty good job of recruiting, though. As bad as we were in his final year, the program was in good enough shape that Pelini could come in and quickly get things on track.
Bo's biggest fault was in the way he left the program. He was ticked he was being let go after being successful (for the most part) on the field, and felt like he was being done wrong. The lack of professionalism he showed in being fired caused division when it happened. However, I don't feel like there were any real long term ramifications for that. He left the program in good enough shape that we were in every game for Riley's first season and were very successful in his second.
I think with Riley he failed to maintain the things that made us successful under Bo. Defense went in the toilet. We were poorly conditioned. Recruiting wasn't very good. I don't think Riley was a bad coach (there is plenty of data points that suggest he can coach football), but I don't think he had what it took to be the CEO of a program like Nebraska. There are very few redeemables about the program upon his departure.
Frost talked a big game, but never did what was necessary to deliver on it, or at least regressed as time went on. He is leaving the program in a place that is terrible from a culture and record standpoint. The one redeemable I do see is that I think he has brought in a little more talent than Riley did. If the next coach is able to walk in and turn around what we have, that might earn Frost enough credit to not be all the way at the bottom. Again, I think time will tell on that.