Talent is independent of rankings, although there is a correlation between rankings (especially at the high end) and expected talent level. Frazier is a talent, arguably the most talented player we've had since Dee/Deron (sans Jereme Richmond). Clearly outplaying his ranking. The rest of the freshmen are about what one should have expected, not really under-performing.
Damonte IMO will be a decent B1G player, given his injury where many expected him to RS, he is definitely not under-performing. Smith is going through some growing pains as a freshman, but not under-performing much either considering his ranking. The reason that some illini fans see Smith as under-performing is because most on this board built their expectations up and placed more value on winning Mr. Basketball than his actual ranking. Many expected a sure starter, and argued that he was seriously under-ranked. Reality is he was not. His ranking was fair.
Leron is not bad this year, definitely not under-performing either under BU. Better than previous years where he played below ranking expectations. He had already been in college for 3 years prior to BU arriving. Was he playing better under Groce? Of course not. It is not that BU came in and Leron started to under-perform.
This is definitely not one of BU's better coaching jobs, nothing to brag about. But he is still a good coach IMO. Can he get the talent? Not sure about that, but is definitely more the case of not having the talent needed, than having talented players who are under-performing.