I can speak from the informal post-bacc perspective.
Advantages:
-cheap, around $1,700 tuition per semester, fulltime
-as a corollary, less financial stress - if unsuccessful in gaining admission to med school, the "sunk cost" of premed classes was truly minimal, vs 30K per year or so for a formal program.
-didn't have to move (no formal programs were near me)
-flexible classes -- I took whatever & whenever I wanted, as the 4 yr school was pretty large, and could decide on the order of courses, which advanced courses to take, etc
-got to meet lots of other students ranging from future healthcare workers to future HS teachers.
-ultimately gained acceptance to med school.
Disadvantages:
-no one "told" me what & when to complete a class, how to prepare for the MCAT, how to fill out the med school apps; premed adviser OK but not great.
-no external pressure to do well; many peers were fine w/b's or so; motivation had to come from within.
-my post-bacc school is not well-known; no warm intro from my post-bacc school to any med school.