I was in Montessori through kindergarten. I do remember feeling a bit disadvantaged socially when I entered 1st grade in a conventional school.

Most of the other students had done kindergarten at my new school and already knew each other. I did eventually make friends, but I always felt like a bit of an outsider since I didn't have that shared kindergarten experience with my peers.

I imagine this would only get worse the longer a child stays out of the conventional schooling system.