If you take the Ten Year War as perhaps the greatest period of any college football rivalry, and look at how evenly matched the two teams were through that period, then you have to give Bo his due.
His lack of national titles had one primary reason: Wayne Woodrow Hayes. Woody's lack of titles through that same period had one primary reason: Glenn "Bo" Schembechler. Were their teams beat up and undermotivated for bowl games through that era? I'm not sure about that argument. What the record does generally show, however, is that the underdog team often won "The Game", and that many of Woody's and Bo's best teams sat at home on New Year's Day.
Whether or not you call him one of the all-time greats, the fact is that he and his teams ruined several potential Ohio State title runs, and we ruined several of theirs. Maybe the best way to rank him is as one of the very best Big Ten coaches -- behind Woody, but definitely ahead of guys like Alvarez, Fry, and Cooper. Obviously, he's big enough to Michigan to have their facilities named after him. In my mind, he's just behind a group that includes modern-era coaches like Bryant, Hayes, Paterno, Bowden, Osborne...maybe somewhere around Darryl Royal?
It's hard in this discussion to compare to old timers like Stagg, Yost, Heisman, etc. so let's leave those out. How does he compare to his contemporaries? Was he the equal of Hayes, Bryant, Bowden, Osborne and Paterno? I don't think so, but he was competitive with them in their time.
To me, the argument that he's one of the very best coaches of his generation is a no-brainer. Clearly, even without the championships he was. His teams were talented, played hard, and were clean. The one stain on his legacy is Voyd.
