Heroku works fine in prod up to the scale of most pre-series A startups. There are a few limitations you have to accept but for us the easy deployment strategy (and existing suite of addons) makes it very easy.
This is true, probably fine for smaller use cases. I cannot imagine doing a production Elixir app without node networking (I do quite a few things with it), and that is a deal-breaker for me on Heroku.
That's really the only deal-breaker though. Docker for Heroku makes it really easy to ship artifacts rather than source which I think is important as well.
Using Heroku (not good practice and I wouldn't use in prod), it would be really quick.