I think the success stories are mostly self-selection: parents who care enough about education to enroll their children in an alternative system will have children who care more. Anecdotical data: in my non-US state everyone (admittedly 3 people) who went to Montessori schools were 3 years behind in every subject as compared to public schools and mostly they did fine with some help, but the difference between who did better or worse was, as you pointed, inherent personality that was completely independent from the education they received.