The big thing that this misses is accessibility.

I'm sure they're accepting Pull Requests.

There are a few accessibility plugins IIRC but yeah it could be much better in that regard.

And the great thing is that they are using their own UI to build the Godot Editor itself.

There is even an alpha build of the editor that runs on Android (and even inside the Oculus Quest).

I wanted to see if they actually handle text well (https://lord.io/text-editing-hates-you-too/) but every one I download to Mac the OS said "This App is damaged and can't be opened"

I'd suggest contacting the authors or just downloading Godot itself as all the UI is the same as those people are using.

Does the UI toolkit support basic text operations like copy-pasting or selecting text? This is a common problem in the alt-UI scene, basic stuff doesn't work after years of development. I don't want to ship an electron app either but at least it has everything everyone has come to expect.

Strong theming as well https://github.com/tristongilley/Godot-Windows-95-Theme

What makes it better than others?

Layout is almost WYSIWYG and easy to get started with a simple download, resulting binary is small, really easy to integrate with an underlying 2D or 3D scene, lots of controls, built in scripting for simple things, great interop with native code for complex things, can deploy to multiple platforms with one project, everything is open source.

Does it support custom UI elements?