Pretty much the same feelings as @hubertlepicki - I tend to think Elixir/Phoenix is easier to upgrade mostly by virtue of not including as much. Also for a 1.x release the APIs of both Elixir and Phoenix are remarkably stable.
Worth noting that the biggest Phoenix apps I upgraded weren’t as big as the biggest rails apps I upgraded.
In general, less magic is happening in Elixir/Phoenix which means less small changes in how magic behaves to bite you. Also that it’s more a set of components working together makes it easy to upgrade first one and then the other as Hubert mentioned. That then often translates to 3 small to medium upgrades instead of one gigantic one.
It also feels to me like in Ruby/Rails you often rely on state of things or some specific behaviour (again magic!) which is not the case as much in Elixir/Phoenix. Especially in the Rails community headaches I’ve had is that lots of libraries are effectively designed as ActiveRecord/Rails plugins which are too often tied to some internal behaviour and then break during upgrades and you have to upgrade all these other libraries as well. I’ve seen that way less with the Phoenix/Ecto eco-system.
All that said, Rails seems to have stabilized significantly I’m aware of nothing too big in 6.0 that’d break existing things but in my opinion Elixir/Phoenix upgrades should be easier but that opinion still has to stand the test of a huge application.
I’d be interested in experience reports from bleacher report… @keathley ?
