What does HackerNews think of ueberauth?
An Elixir Authentication System for Plug-based Web Applications
ueberauth is one of the most fully featured with support for a ton of different authentication schemes (https://github.com/ueberauth/ueberauth)
But there are lots of other things that integrate with Phoenix / Plug (https://github.com/h4cc/awesome-elixir#authentication)
For example, when I last read through the codebase it was common to find features that simply wouldn't work, because the code referenced hardcoded parts of the sample application.
You'll also going to run into a lot of issues trying to migrate off of Coherence if you ever need to support anything other than form based username / password login.
I think it really is worth the effort to go with Ueberauth [0]. You'll need to do more work upfront, but the maintainability gains will quickly pay off. You can even use :ueberauth_identy [1] to provide username / password based auth without too much trouble.
If you're using Absinthe you can create a easy middleware for auth as well.
If you want something like Devise from the Rails world, there is also Coherence. https://github.com/smpallen99/coherence