While the Vestibular is generally considered to be a fair and unbiased system to select students, there are controversies.
There is some criticism to the alleged standardization of the high school curriculum for the whole country to match the Vestibular agenda. As most types of academic evaluation, the vestibular suffers from the same limitations as a regular test; that is, factors such as stress come into play. It also seems to favor candidates that come from a wealthier background, who had access to better and deeper education in private schools, as opposed to the comparatively poor public high school educational system, although many university boards claim that it doesn't matter which background the candidates have, since their Vestibular aims to select the best students based solely on their knowledge.
To ease this problem, some universities such as the Federal University of Minas Gerais, provide a 10-15% bonus for students who received their middle and high education in public schools. There is also the alternative of taking classes at cram schools called cursinhos. Those are revisions of Vestibular subjects, given through periods of 9 months (the regular year-long one), 6 months (the semi-intensive one), and 3 months (known as intensive). These "cursinhos" are given usually for the most prestigious schools and can be done after concluding high school or as a complement of the last year of it.