While easier than before to do server side rendering, it's still non trivial to do a react app that preserves SEO and renders on the server if your app isn't already a nodejs app.
It would require running either V8 to render on the server behind whatever language your backend is already written in, or calling node to serve your backend, relegating your existing backend to an API (and tying the backend rendering to your API along with the Frontend to use the same API isn't simple either).
It would require running either V8 to render on the server behind whatever language your backend is already written in, or calling node to serve your backend, relegating your existing backend to an API (and tying the backend rendering to your API along with the Frontend to use the same API isn't simple either).