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).
What exactly makes you say that Rails is a clear win over Phoenix for CRUD stuff?
To you use typespecs and Dializer? (I haven't looked into those yet.)
My experience is that a good share of bugs is avoided thanks to immutability, but I still get nil-related issues now and again.
In Ruby, if I'm trying to find the source of a dynamically generated method in a dynamically generated class, that usually requires me to run add a debug statement, run the program, and see what's being passed. Even if I have a stacktrace, I often find it difficult to understand what the code is doing without running it.
I find Elixir code easier to read, and grepping my way through the code base is often enough. And to be fair, a debugger is often less than useful in macros involving AST & when dealing with a process that got a random message from who knows where.