From my experience supporting Rails app longer than a month becoming more complex due to a lot of meta-magic (method_missing and friends, abused by rails itself). Elixir/Phoenix feels exactly like Rails with these issues fixed. Longest living production app is ~8 years now, doesn't feel like legacy at all. Upgrades are easy. Easy to figure out what's exactly happening right there.

Also, resource consumption/optimization. I noticed i need to do something about it on first week of development, which is sad (and spends dev time on non-relevant stuff). Phoenix designed to avoid it (including stupid stuff like N+1 problem)