I know of these two projects that offer some guidance to combining RoR with React:

https://github.com/shakacode/react_on_rails

https://github.com/netguru/react_webpack_rails

Basically, you introduce webpack as part of your dev/build pipeline, it interacts nicely with the asset pipeline.

My current preference is the 'browserify-rails' gem. I set up a package.json file to manage my javascript, but I let the regular asset pipeline handle the css and images. I tried webpack for a while, but it seemed redundant when I already had the asset pipeline.

I also like to add 'babelify' to my package.json and 'config.browserify_rails.commandline_options = "-t babelify"' to my application.rb to get ES6 goodness.

Ruby Gem https://github.com/reactjs/react-rails works fine.