I'm only comparing development experience and, so to say, the conceptual environment. Verily, Rails can be a pretty good and compact way to serve backend APIs

I'm only comparing development experience and, so to say, the conceptual environment. Verily, Rails can be a pretty good and compact way to serve backend APIs for a React-based frontend. But the API boundary is the "narrow waist", its relative neutrality allows to switch backend and frontend implementations easily, or even to mix them. I've seen a React frontend seamlessly consuming APIs served by Python and Rust services, and it's hardly possible to spot which endpoint is served by what in the frontend code. Unlike on the frontend, FP approaches on the backend are not (as) widespread. Established imperative frameworks (Django, Rails, Spring Boot, etc) are still the huge majority.