My oldest daughter attended private Montessori until age 12 when she transitioned to an exurb public Middle school in an extremely high income area.
Transition was easy because the Montessori middle school she was going to was new and hadn't gotten their legs yet, so had a pretty lackluster program. Adding to that the lack of extracurricular and club/team opportunities, and small class sizes, Montessori method starts be become a hindrance to learning the complex social dynamics you need to survive IRL.
My other kids transitioned to US public education at 8 and 10 respectively with no issues
The most difficult transition was for the 8 year old, simply because her personality fits the more loosely structured method of Montessori better, however this faded pretty quickly
Should be noted also that my kids are extremely naturally gifted and generally live in the "AP/Honors" world, so would most likely flourish anywhere. YMMV