Parent of two long-time Montessori kids here. We have been through both public and private Montessori schools. Unfortunately, the public one did not work out because they were heavily constrained by the district's requirements, so the Montessori aspects were very watered down.

What I like about Montessori is that if a child finds a "work" that interests them, they can really focus in on it until mastery instead of feeling pressured to follow a specific curriculum on a specific timeline. I also like the de-emphasis on grades and testing. These are distractions from learning IMO. As for academic progress, one of my kids picks things up early and the other picks things up late. It just depends on the child. We aren't yet to high school, so I can speak to that transition. I'd be lying if I said it didn't worry me a little, though, mostly because I think of high school as a stressful place and my kids are currently in a very low-stress place.