Just letting you know that I have finally got round to testing uibuilder with webpack.
I've put together a quick outline of the best way I've come up with so far and added that to the WIKI.
I know that some folk have had some issues with uibuilder and webpack which is why I've been investigating. I can now say confidently that uibuilder does work just fine, at least with webpack v4.29+. If you are having issues, please try out the WIKI page first so that we can work out where the issue lies. The basics definitely work fine.