I strongly agree that there should be a little better support for external editors. Similar to @Simon and @oskarsh I have really fought the built in editor when it comes to just getting code written. I seem to gravitate back to the built in when debugging and working in UI.
The auto-autocomplete is also extremely unreliable for me. It feels like it stops working completely pretty often.
Honestly, being able to avoid overwrite-on-compile and watching for script changes / auto compile would be enough for me. The regular JS syntax highlighting in Sublime, Atom, VS Code are all much better for me but could easily be community addressed if someone wanted something more robust.