First things first, I don't like the hire, but let's take a look at his career as opposed to rank speculation and conjecture.
He was first a DC at Western Carolina in 1997. I don't think it's really fair to judge that as part of his body of work, but if you do his defense wasn't bad. The most they gave up all season was 30 points, ironically to Paul Johnson and Ga Southern. Otherwise, he gave up 28 points (loss to ETSU), 25 points (win v. the Citadel), 24 points (loss to Chatt). The rest of the games the defense held the other team to under 20 points, including a shutout against VMI. WC was terrible that year, breaking 20 points only three times, so the offense wasn't doing him any favors. Overall, a pretty decent defense although I don't think he gets much credit for it (positive or negative).
Then he was DC for us from 99-2001 (not looking at his years as an assistant). 2000 and 2001 were pretty good defenses for us. We were a near mess in 1999. 100th in total defense (413.8 yards per game) and 93rd in points (30.3 per game).
In 2000 we were 12th in rush defense (94.5 ypg) but last in pass defense (285 ypc), and 70th in total D (379.64) - although we improved about 40 yards per game. Defensive scoring vastly improved, going up to 21st nationally (19 points per game).
In 2001 we were 23rd in rush defense (117 ypc), 53rd in pass defense (216.25 ypg), so again an improvement over the previous year - 32nd overall in total defense at 333 ypg. Scoring defense was worse, 37th nationally at 22.3.
Overall, the defense arguable improved every year he was here from 1999 to 2001.
I liked him a lot at Duke (and was up in the area when he was coach). Even though his teams were terrible, he seemingly recruited good there (for Duke at least). For instance, in 2005 they landed a 5 star DT, a 4 star DE, and a (can't believe I'm typing this) 4 star kicker. That's on top of 8 three star guys (which for Duke is a lot). Similarly in 06 they landed a four star DE and a four star RB and had 9 three stars. 07, Roof's last year, they got 7 three stars. Roof was fired from Duke on November 26, 2007. Before then, for the 2008 class, he had another commit from a 4 star CB, and four more 4 stars. Renfree committed in January, but I don't know how big Duke was on his radar while he was committed to us.
When he was DC at Duke (arguably all that's relevant), they improved from 113th to 58th nationally in total defense from 2001 to 2002. Roof was basically promoted to head coach because of how decent the defense was.
After head coach at Duke, he was DC at Minnesota (2008), Auburn (2009-2011), and PSU (2012). He inherited the worst defense in the country at Minnesota, but improved by 135 yards in total defense. Scoring defense improved from 109th to 61st in the country. Gophers went from 1-11 to 7-6. Not stellar numbers, but decent improvement.
Auburn was a mixed bag. He won a national championship, but Auburn was a mess in 2009 (68th in total D and 79th in scoring D). They were mildly improved in 2010 (60 in total D, 53rd in scoring D), but regressed in 2011 (81 and 79).
At PSU he had the no. 29 total defense (350 ypg) and no. 16 scoring defense (19 ppg)
In sum, actually looking at the numbers I don't think he's as bad a hire as I thought he was. He stunk up the joint at Duke and Auburn. But he's shown capable of producing a top 30 defense with us and UCF. I don't think we will end up with stellar defenses, but I like his aggressive nature (hopefully will lead to stops, which is really what we need) and his hire should translate into much better LB play. I'd maybe give it a B-.