I'd love to know more as well! I literally just started "school shopping" for my DS.
My MIL worked at a school the year after they broke off from Montessori. They still used a lot of their methods, but they felt overall that it was too restrictive. For example, there's a specific way to teach a skill in the Montessori way and if a child struggles with that method they cant just switch and try to teach the child a different way. So she is against Montessori. That was... 23 years ago, though, and just one school.