I'm currently using absinthe which enables a graphql backend on the elixir stack with apollo on the frontend. its been working fantasticly well for me so far. Its backed by postgresql and the hardest part so far has been making sure to batch sql queries for complicated graphql queries.

I haven't used apollo-server but I wrote nodejs code for 8 years and I wouldn't trust it to scale without a lot of effort being put into your DevOps story.