What makes this worth paying for, without considering the technical details (like maybe it's a bit faster for some stuff than Godot or Unity) is just Ruby.

Yes, most programmers are able to learn other languages just fine, but we all have preferences. And people who prefer Ruby usually tend to like Ruby a lot.

Apart from being Ruby, the simplicity is a major selling point for me.

(Yes, I have bought it, though not had time to use it much lately)