Um, again, medical school
is standardized. Even with slight variations (ex. PBL vs. traditional vs. integrated curricula), you still learn the same information. Not only that, you have Step 1 after the basic sciences ensuring that everyone ends up learning the same information. And in the clinical years, from what I understand, you have standardized shelf exams, etc, built in to establish minimum competency.
Additionally, even with the variances in curricula, there's actually literature that shows no difference in board scores, etc, between PBL and traditional, etc.
Here's a link to the accreditation standards set by the LCME:
http://www.lcme.org/functions2010jun.pdf