disclaimer: I am the current maintainer of both the mongodb and mongodb_ecto packages.
Unfortunately the MongoDB support in Elixir is a little lack luster. I would attribute it mostly to the fact that largely only a single person seems to work on it at any given time. There are of course large contributions that come along. With that said, replica set support semi-recently landed in the mongodb package. Unfortunately there were a number of issues with the initial implementation. So I am slowly trying to fix them. There are also a number of features I am wanting to address. Unfortunately I only have so much time in a day.
With that said, I am using it in production at the moment. So if you, or anyone else of that matter, have any questions about getting started, please let me know and I can update documentation or something to make it more easily understandable.
Edit: There is also #elixir-lang channel on freenode or the mongodb channel in the Elixir community slack group where I hang out if anyone has any questions.
