Very small anecdotal experience, but I was in Montessori school for grades 2-3 (in the late 80s) and it ruined my math education. When I should have been learning the standard ways to do basic math, they were teaching me nonsense like "skip counting" with rhymes. Despite coming from a family of people very good at math (which also put a focus on math education at an early age), I fell behind and never caught up.