Tilmon drops because he did not impress in HS games with enormous size and athleticism advantage. I do not think he's ranking is illini-related. Frazier did not drop in Rivals as a illini commit and re-commit.
2015 class is about rightly assessed by the recruit website, except DJW is overacted. TJL in 2016 is underrated. HM, KN, LB were top 100 recruits and they are indeed good. Mav, Colbert and Finke were not there.
So I would say illini players were assessed generally correct in the past few years. If there is a team get biased of recruit ranking in B1G, that is Wisconsin.