Elixir seems cool. RoR and React Native used to be the bread and butter given the bustling community. I imagine the community is still there and I'd still recommend anyone wondering about this question to just use RoR+RN. But I think Elixir has been a really interesting piece of tech that now has native mobile app dev capabilities[0] though IDK how it compares in ease-of-use and community support:)

[0] https://native.live/ -- HN Discussion @ https://news.ycombinator.com/item?id=32694668