My biggest concern is payment processing libraries are waaaaay behind in Elixir. For example, Stripe doesn't have an official Elixir library (but they do for Ruby and Python), so that leaves you with a bunch of community driven packages that are all in various incomplete states.
Likewise PayPal has an official SDK for Ruby and Python but nothing for Elixir and it's very slim pickings on the community side of things here, to the point where I would feel uncomfortable using any of them.
But other than that, things have been good so far. A lot of common CRUD problems are solved, for example with pagination Rails has Kaminari and Elixir has scrivener which took like 2 minutes to set up. You'll find well maintained libraries for a lot of common web dev functionality. Plus, things like Turbolinks works great with Phoenix if you're into that.
Generally speaking you'll end up writing a little more code with Phoenix than with Rails, but I find it much easier to grasp what's happening in Phoenix because there's so much less magic (but not so little that you need to implement a million things yourself).