I'm pretty much all in on Phoenix, but I suppose a good reason to stick with RoR would be that 1) it's easier to find (experienced) devs, and 2) there are (still?) many more packages available which can make quick development easier.
Have you seen anything different? Most enthusiastic vendors I’ve talked to have moved away from elixir due to low adoption and demand.
Tiobe, google trends, etc tell the same story.
I’d love to find evidence to the contrary but the low numbers of elixir devs seems to be both a supply AND demand problem... ie lack of traction and interest.
In the latter case, I feel confident enough about Elixir that I'd train someone to use it rather than opting for a more 'mainstream' stack.
I suppose if getting new developers on-board asap is a concern, Elixir might not be the best choice. But personally if I ever were to build a product that would eventually need more programmers, I'd pick Elixir even just for the 'filter' effect. It's easy enough to pick up for a good developer, but would filter out many of the shittier devs I'd encounter if I went for PHP/Python/Ruby. and the benefits are worth it to me.
All that aside, while I am not confident that Elixir will gain much traction, I think there's a small chance LiveView and Telemetry, among other projects, might divert Elixir from the path that, say, Clojure, seems to have taken when it comes to traction.