I am sure Mack Brown factored in heavily that Collins is a pretty good recruiter who knows Georgia extremely well, as well as solid work as a DC at Florida and MSU. When he interviewed he apparently had watched a ton of film and was very well prepared/had a vision for how he would improve the defense. And we know he is a salesman.
Collins failed spectacularly as a HC. I don't think he has the focus for being a HC and he had a lot of kooky ideas that got away from focusing on what's really important—fundamentals, discipline, toughness, effort, and managing the team's psychology/identity. Instead he focused on dubious ideas such as branding, ATL, his vision for being a developmental program, etc. Not very savy with media, pressers, etc.
I think he could succeed as a DC at UNC where his scope is limited. What I think could be a real concern is Collins trying to poach GT players via the portal.