Phoenix is really a very small framework, and its codebase can be easily understood in just a couple of days. Phoenix being "large" is a common (but unfounded) criticism that seems to stem from Elixir's superficial resemblance to Ruby.
Beyond that there's barely any resemblance.
I really do like the coding convention though that lack of parenthesis can indicate a function meant to be a part of a DSL.
...Gods, what a relief. Especially the automatic formatting on save is just a lifesaver. At certain point I would not mind if there is one formal syntax enforced like Go does.
Ecto is practically the only library I use where I don't pay attention to parentheses. And the formatter has me covered afterwards.
I'd be interested to know what you believe the ups and downs are, aside from speed.
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.