I found the lack of 1st party auth system annoying. Phoenix, for example gives me a nice starting point (even magic links soon). Adonis for all its features, turns me to what I hate. The JS auth implementations.
I'm not familiar with Elixir/Phoenix, but from reading other recommendations in this thread and after having a quick look, I need to take a closer look :-)
"AdonisJS ships with a robust and secure authentication system you can use to log in and authenticate users of your application. Be it a server-rendered application, a SPA client, or a mobile app, you can set up authentication for all of them."