You can sprinkle react components into your regular templates which is quite nice. A full blown SPA feels like way overkill (for my own projects, at least) when Rails handles forms and navigation so much better. You can include packs on a per page basis so it doesn't slow down / bloat the rest.

Completely agree, I run a Rails 6 app which provides a JSON API. This is then used by Create React App app running in the /clients folder at the root of the Rails application folder. Keeps it separated nicely.