For one thing, “excellence” in the Harvard admissions process—and at Harvard—has a lot less to do with virtuous character traits than with an ability to game the system. By placing a premium on students who go above and beyond in extracurricular realms, Harvard has attracted a number of truly incredible people but has also encouraged a high school arms race wherein kids cram their schedules with activities in an attempt to attract admissions officers.
By selecting for this kind of behavior, the admissions process doesn’t encourage real excellence, but, to use the novelist Walter Kirn’s term from his hilarious book and essay “Lost in the Meritocracy,” “aptitude for showing aptitude.” This may well be of use in students’ careers after college, but it is orthogonal if not antithetical to the goals of a liberal arts education.
I think that these factors are also an issue when you evalaute education at any level. People compete hard to get into top schools -- the same skills that they use to "game the system" also tend to ensure decent post-Harvard outcomes.
This makes evaluating the actual contribution of the education at Harvard, itself, tricky (to say the least).
h/t: Felix Salmon