Guys usually end up underrated because of what happens after they leave UK. Meaning it's easier to forget about how good a guy was in college if he never has a pro career.
Some of the players mentioned in this thread, I'm not so sure about. Bogans? OK, underrated how? Everyone remembers him, everyone knows he was a really good player. It seems like some people want to elevate him into an all-time great, though, and I don't buy that. He was a good freshman on a bad team (worst UK team between 1990 and 2006, and one of the worst offensive teams in school history), he had an excellent sophomore year, he had an inexplicably terrible junior year, then rebounded to be a lower-level All-American as a Senior. Nice career, but not an all-timer just because he was around 4 years and ranks high on the scoring list. Bogans was a far better player, but I'd add Ed Davender to a similar category.
Some guys get mentioned so much in these discussions that it becomes impossible to keep thinking of them as underrated. Anthony Epps leaps to mind. One of my all-time favorites, but if everyone always mentions him as being underrated, he ceases to be so.
I would nominate Kelenna Azubuike. UK's talent level was seeping away from 02-03 through 03-04, yet UK managed to put together excellent years in 03-04 and 04-05. Chuck Hayes was probably the biggest part of that, but he gets mentioned all the time. Azubuike rarely does, but he was a high caliber player on 2 top 10 teams.