The worst defenses by recent NCs are 2019 LSU, 2014 OSU, and 2010 Auburn.
They all averaged around 22-23 points allowed per game and around 350 total yards allowed These are not bad numbers in a vacuum, but for a NC team, they're dreadful.
.
This year, USC's defense allows 28 points per game and 415 total yards. So I'd posit that the Trojans were never actual NC contenders, as their defense was far too poor. A loss to UCLA, Utah or vs a playoff team was virtually guaranteed.