Montessori for 5 years, myself. I mostly learned how to learn, and the struggles later were that my education was well beyond that of my public school peers when I switched. I found learning by rote to be a waste of time, and continued to learn in my own ways.
As a result, I had amazing test scores in public school, but bad grades overall because I felt that the homework was a waste of my time.
I still appreciate my time on a Montessori school though.
My experience as well—I went to a Montessori school up through Kindergarten. I absorbed enough that I could coast without learning any new math until 3rd grade, when we started doing long division. My teacher either didn't notice or couldn't help when I started suddenly struggling. It put me off math for the rest of school.
It's still a sore point for me 30 years later. I do a lot of graphics programming, and I know I have the type of mind that can handle complex math, but I missed that fork in the road.
As a result, I had amazing test scores in public school, but bad grades overall because I felt that the homework was a waste of my time.
I still appreciate my time on a Montessori school though.