Criticizing Phoenix for being the Rails of the Elixir ecosystem is particularly ironic given Phoenix's creation and design was heavily motivated by incorporating the lessons from Rails' successes and failures.
What are the failures Phoenix addressed from rails?
Phoenix has less 'magic', for one. While it sometimes does mean there's a bit more boilerplate, I think it strikes a nice balance, and the 1.4 actually removed some magic.
Another one is the ORM, Ecto. It sticks much closer to SQL and I strongly prefer it over ActiveRecord. Ecto also includes the concept of changesets, which is a wonderful way to deal with validating user data and definitely an improvement over the Rails approach.
And finally, while I wouldn't necessary call it a 'failure' on Rails' part, the channels functionality in Phoenix is awesome for the more real-time apps that we have these days, and Phoenix is generally quite a bit faster in general than Rails.
I'd be interested to know what you believe the ups and downs are, aside from speed.