I am a long time Elixir user and I also find the VSCode experience a bit frustrating ; sometimes the completion did not work, for hard-to-tell reasons (I'm switching computers often).
I have seen a couple of issues that have been worked-out.
In particular, it's nice to have completion in ".exs" files!
One thing that's been bugging me recently is completions in eex/heex template files. It seems like a tough problem to solve, since you need reasonable completion for html, heex syntax, and arguably also completion on things like schema fields for forms and then components that the project defines.
But man, if I could have a smart editor for those templates I would be like... three times as productive on my little side projects.
I have seen a couple of issues that have been worked-out.
In particular, it's nice to have completion in ".exs" files!