Well, for starters, it's results-based. Those 'winning schools' have less loses, especially embarrassing losses, to pinpoint. If that weren't the case, maybe you'd hear the same criticisms Harbaugh is getting -- tuning out their crazy yelling coach, for example.
Bo's defensive coordinator rep is also pretty well-cemented. So when teams of all different calibers and talent levels are either moving the ball freely or just steamrolling through the Blackshirts, the thoughts that come to mind aren't "There's a defensive coaching deficiency here" or "Gosh, Nebraska just doesn't have the talent to match up to....Minnesota? Iowa? Wisconsin?"
Finally, the style of play Nebraska has been known for recently has to be mistake-prone, tight, Jekyll-and-Hyde. The roller coaster seems to be reflective of the coach's own emotional roller coaster. Belichick gets heated, he's gruff with the media, but both he and the Patriots are famously and emphatically even-keel, both in message and in result. And not being able to play loose, that's an easy line to draw from the reactions we've seen from several players to getting chewed out on the sidelines -- something that's not wrong for a coach to do, but whether he's being effective at it or not is another story.
I really feel it takes some mental gymnastics to get back to the point of saying, "I have total faith in Bo's demeanor as it pertains to gamedays and the NU brand", but then, I can't fault any Nebraska fan for feeling predisposed to that direction.