The creators of Phoenix didn't want to write their own code to manage Javascript and CSS (e.g. rewriting the Rails "asset pipeline" in Elixir), so they just integrated Brunch [1], which is built on Node.js and NPM. I think this was an excellent decision. (Node.js is only needed during development and deployment of your assets. It is not used in production.)
That is still pretty awful imo - think quite a few of the people considering Elixir and Phoenix are doing so explicitly to get away from having to Node.js close to their computers in the first place.
I personally don't consider it a problem if I don't personally have to touch it. Phoenix does that reasonably well.
I don't see the problem. Why not use Javascript toolkits to package, and develop in Javascript?
I can understand languages not self-hosting their compilers, but I'd at least like it to self-host its toolkits---bundlers, packers, preprocessors, etc.