guys like McMichael, Ritzie, Dalton, and to a lesser extent Gainer (though he was injured most of his year at UNC) got their cup of coffee in the NFL due to athletic traits, not necessarily because they were good football players. McMichael has spent time with three different teams the past three years and hasn't played a snap for any of them. the back ends and practice squads of NFL teams are full of that kind of player, with coaches thinking there's maybe a chance they can harness those athletic traits in a way that hasn't happened yet - there are only so many good players available, after all. it's not at all unique to UNC, but we've probably had more of that kind of guy recently because of Mack's great recruiting classes that didn't pan out due to him being a bad coach.
i saw somebody once say that there are three kinds of hidden gems in the NFL draft: injured players, small-school/HBCU players, and great athletes who weren't very good players. when you get to day 3 and the hit rate gets that low, those are the archetypes that give you a slightly better chance of getting a player who will stick on the roster.
the only surprises to me on your list are Storm Duck being a multi-year NFL starter (for a dysfunctional organization, sure) and Prince spending actual time with the Ravens, which really does defy explanation.
as for mckethan, ezeudu, and asim -- asim was an awful, awful player as a junior. unc got an actual OL coach instead of stacy searels for his senior year and he revolutionized his play basically overnight. and mckethan was only okay, he's another who was drafted for traits. and honestly, an OL is only as good as its weakest link, and the guys we didn't name on that line were pretty bad.