I’ve also been using it for years with Rubymine and it is stable. You may see the IDE error dialogue when you first load, but it doesn’t seem to affect any operability and does not ‘crash’.
I generally use the test runner in the IDE and have not had any real problems…except when you update Elixir and it seems to get stuck on an older version of Hex if you are using ASDF. Then it complains about the mix.lock file being built with a newer version of Hex. The solution to that is to delete the .mix folder under .asdf and it will reinstall Hex and Rebar on the next test run, then be fine. There is an issue open on the plugin website for this. Honestly, in the last 2-3 years, that’s happened to me only twice. If you ever see that, just wipe that hidden folder and you’ll be fine.
Otherwise, I’m quite happy with it. The plugin is kept pretty current with Elixir and Phoenix. Rubymine is great for general website development because of the tools that come prepackaged…version control with excellent conflict resolution on merges, debugger, test runner, terminal and, most importantly, the database tool (which doesn’t come with IntelliJ-CE). Very little configuration needed and it pretty much works out of the box with the plugin.