Without having researced it, I can't say what facts exist. But if I were doing it I would research the Strength of Schedule (what were the rankings of the teams Nebraska defeated at the time they were defeated), the opponents' Stength of Schedule, average margin of victory, and such. I don't know where that information is kept - I would think the NCAA would have it somewhere.
The trouble with that is you have to do the same for the other so-called "greatest teams of all time" - which means identifying which those are. You might want to start, therefore, with an Internet search of lists of the top 10 teams of all time. Take those 10, and apply the same criteria as you do to Nebraska, and then compare them.
If, by some quirk, Nebraska comes up short in a statistical measure, don't let that dissuade you. If a team "ranks" higher in some category, see if there aren't factors that off-set that. For example, teams from older eras played fewer games - which made their chances of being undefeated better, and therefore their "ranking" on this would not be as high as Nebraska's in the "modern" era.
Just some thoughts.