Useful gems that work well with rails 4
neblit.com
neblit.com
If you compare setting up a new project in Rails 4 that lets users authenticate with Github, with doing it in Meteor, you'll see what I mean.
It depends on what you want to do of course. A lot of the following behind devise is due to how well it deals with all of the ceremony involved with standard authentication.
Everything from sending out e-mails, password resetting, failed login constraints and all of that stuff. It's nice to be able to hook in a gem and perform minor configuration to get all of that with no fuss.
In your example of only having to auth with github then yeah, I wouldn't use devise either because it's so basic. Omniauth is what I would use in that case. I would see that as a toy example too because more often than not you'll want much more than just auth'ing through a third party site.
I'm sure that, given enough time and patience, I could have figured out which methods, controllers and models I needed to write, and what to put in the config files, but it felt like a major regression convention-over-configuration. Compared to meteor (type 'meteor add accounts-github' and add {{loginButtons}}) it was really painful.