I don't have any experience with Elixir / Pheonix.
From comments I read, even things like deployment change frequently in Elixir, and major libraries are created or abandoned.
It's just still a very new ecosystem.
In 5-10 years I'm sure the rate of change will decline.
Then there is a stack like Node where constant change seems to be just part of the culture.
Rails changes too, but not as much.