RoR is a great stack! But I am not a big fan of rendering HTML server side using traditional templating engines. Also if you develop using just RoR and vanilla JS or jQuery, the front-end code may get unmaintainable once your project grows.
According to me Angular and React are two great libraries and should be used with backend frameworks like Rails. You can build your app using Rails and Angular and use the same APIs (or may be some tweaks are needed) for mobile clients. But the only downside is that you need to pre-render your pages on the server to be SEO friendly. You can either do that yourself or use a service like Prerender.io.
On the other hand if you choose to use React and RoR you'll have best of both the worlds. You can build APIs in Rails and use both client & server side rendering. I think isomorphic apps are the future and should be used whenever possible. This fixes your SEO issue and also gives you power to use same APIs for mobile clients.
How to build isomorphic apps using React - Found this interesting.
How to use React with Rails - Another interesting article
Hope this helps!