This is not true. My wife has taught in a Montessori classroom for over a decade, and the children who get the most out of it are the ones who would be labeled as "problematic" in other environments.
I went to a Montessori school in Sweden for 10years and while i never was a problem child. I have always had problems with learning things, taking long time and lack dicipline and organization. I never got the most from it, instead the school got the most of my will to do everything but learning school subjects.