Elixir is definitely interesting and one to keep an eye on. But I'd guess that for startups to use this, is still very risky especially at this stage. You may likely run into scenarios where you need some library/package and you can't find one for it and that could be a potential deal breaker for some startups.

I'm interested to hear though, can anyone care to comment on their experiences with Elixir's package ecosystem? How much developed is it and/or how much more to go to be suitable for most major projects?

I think that the package ecosystem is well developed, and getting better every day. For the most common things, there's already packages out there. You want to talk to a database? Use Ecto. You want to check code quality? Use Credo. You want to respond using JSONAPI? Use ja_serializer. You want to talk over HTTP to something? Well there's two main options out there (HTTPotion and HTTPoison).

If you can't find a package then you could try looking it up at https://github.com/h4cc/awesome-elixir or asking on the elixir slack (https://elixir-slackin.herokuapp.com/) or on the #elixir-lang channel on Freenode.

After all that if you can't find a package that suits your needs then it's very easy to create one. "mix new (your project)" and away you go.

That's what I did with my elastic package (https://github.com/radar/elastic) when I couldn't quite find something that suited my exact needs. There's Elastix and Tirexs out there (those are the ones I know of) but neither really suited what I needed, so I just built my own.

I don't know how it could be a deal breaker if a package didn't exist already. I'd encourage you to build whatever you don't find already existing and contribute back to the community.