WEll there is some accuracy to them.
Vlad Guerrero was a consensu #1 overall prospect. He certainly showed it. CHris Paddack was ranked in the top 40 by 2 of the ranking bureaus and #66 by BA, and he absolutely performed well.
Sure, every now and then an unranked prospect is great player. No one ever ranked Paul Goldschmidt anywhere, for example. No one ever ranked Youkilis anywhere, either. And Albert Pujols was ranked one time and ranked #42 by BA. So in an interesting development, BA thought in that one year, 41 minor leaguers were better than Pujols, but in reality, there were not 41 major leaguers in MLB history who were better. (Of course, Pujols was in A-ball at the time and then jumped to the majors.)
But there is a strong tendency towards better players typically being higher ranked prospects...