I don't follow. You can write other modules in different languages. The data sources are language agnostic. Nobody forces you to do use a specific programming

I don't follow. You can write other modules in different languages. The data sources are language agnostic. Nobody forces you to do use a specific programming language in the first place unless the platform is close sourced. The point is that with a close source BaaS you have no control on the tools or costs or availability(i.e. deprecation policy) of the service.