Honestly I hate the react-rails gem. Whoever was maintaining it kept doing things at the whims of issue requestors. I remember one instance where they broke our apps with an upgrade because someone decided it would be a good idea to shim a definition for `window` into server prerenders. All because an issue creator who didn't really know what they were doing supposedly "needed" that to get browserify-rails working.
So we made this: https://github.com/revelrylabs/execjs-rails. That handles the server-side rendering for us. We only use react-rails for its assets, never for its helpers.
And here's our own (always-evolving) layer on top of execjs-rails: http://toolkit.revelry.co/pages/core.
Examples are written in dusty JavaScript, but we've successfully used revelry_core with sprockets-es6 several times now.
EDIT: Still no modules yet. It doesn't help that Josh Peek deleted the old sprockets-es6 repo and all its issues before passing off maintenance to someone else.)