However, does it have a mature ecosystem now ? What about deployments and tools around it ? Does it work with generic web servers like Nginx or does it run its own http server ?
The deployment topic is wide, so the answer will depend on your favorite ways to deploy. If you are using Heroku or another PaaS, it has been very straight-forward since ever. If you want to ship a self-contained executable or a Docker image, the language has standardized on the tooling for assembling those in the last year or so (they are called releases). It was always possible before, just not standardized at the language level.
It runs its own http server but you can easily put it behind nginx, websockets included, by using the reverse-proxy settings. Here is an example article: https://dennisreimann.de/articles/phoenix-nginx-config.html
The tooling and developer experience in Elixir is first class and the ecosystem is highly mature.
I've done plenty of Django and Flask and I'd rather work with Phoenix. I still like Django plenty but only time I'd pick it is if I think the admin might save me copious time or if there is something else tying the project to Python, such as machine learning.