The externs, and especially the editor plugin, are still new, so if you find a bug please open an issue on github.
If you want to request a feature of discuss the project there is this topic and github discussions.
The externs have been updated for Godot 3.4 (api documentation) and are available on haxelib.
(EDIT: there was a small issue and I released a patched version 3.4.1 on haxelib)
The release also include improved type safety for signals and actions, better interoperability for arrays, the editor classes and @:tool metadata. With scripts no longer requiring to be attached to a node to be compiled.
The @:onready metadata allows you, similarly to gdscript’s onready, to delay a variable initialization until the node is ready.
@:onready var myNode = getNode("Path/To/Node).as(Spatial);