I'm a fullstack JavaScript developer. I'm not too fluent in Rust and Ruby. I would be more efficient poking and hacking a nodeJS codebase. If I treat the whole codebase as a black box there is no advantage for me in using an open source project.
No, but an API is a leaky abstraction. you'll notice slowness, you'll encounter bugs, you'll see shortcomings in the data model used. All of those flow from the choice of language used on the backend. I'm not claiming that any one language is better than another, just that the leaky abstraction means the choice of language will have user-facing consequences.