OMG thank you! So many times I've wished for a nice way to leverage React UI components within Rails without the nuisance of APIs, client-side state, etc. This appears to fit that bill nicely.

Are there any glaring reasons one would not want to use this library?

Mutagen is what you're looking for!

- Local and remote copy of files - Two way sync

I've been using this for years, editing locally with Sublime, while running everything on a remote Linux vm. Because it's two way sync, anything created/modified on the server is also sync'd back to your local filesystem. I end up syncing most of the remote $HOME so I can easily fire up a new VM, sync, and go. It really is fantastic.

https://mutagen.io/documentation/synchronization

I have yet to use this for anything, so I can't vouch for it's quality, but it seems to be exactly what you are looking for.

http://www.keycloak.org/

Website blurb:

Add authentication to applications and secure services with minimum fuss. No need to deal with storing users or authenticating users. It's all available out of the box.

You'll even get advanced features such as User Federation, Identity Brokering and Social Login.

If you're okay with AWS, check out https://convox.com/