Some of Phoenix's best features, like presence and websockets, break the 12-factor application rules. State is stored in the app and nodes require linking together. Most container systems are not set up for this and you have to rely on something like etcd to do orchestration between nodes. That or just move the state back to redis.

I think the positives outweigh the negatives. It's nice to replace a lot of moving pieces in a RoR application with just Elixir and Phoenix.