This is a great article that explains, from the research, why no one method is a guarantee of bilingual success. When I do parent seminars we talk about this a lot. Being an OPOL family doesn’t automatically mean your kids will be bilingual. Having a minority language at home does not automatically mean your kids will be bilingual. Outside of true bilingual communities, successful child bilingualism happens when you pay attention to ensuring adequate quantity and quality of input, and provide meaningful opportunities for language use.
It won’t happen by itself – you need a plan!

A question of primary importance concerns the type and amount of language input the child will receive, mainly from his/her parents but also from other sources.

The Languages you Speak to Your Bilingual Child | Psychology Today.