The fact that Facebook themselves will never be able to replace their native iOS and Android apps with RN implementations says it all imo.
They haven't rewrote in RN because it's unnecessary. Its not worth the engineering resources. There is so many things on the global product roadmap to build then rewrite something that already exists and works fine.
Like all software, RN is great for certain use cases. Just because someone isn't using it for X doesn't mean it also sucks for Y.