Horizon docs have this explanation: https://horizon-eda.readthedocs.io/en/latest/why-another-eda...
I'll have to find a simple project to test this out on though, because as you say, It needs to be a huge improvement on KiCad to warrant a switch.
Horizon misses nothing (for me) now and has a beautifully efficent UX compared to Eagle and KiCAD. Their library concept is also well thought through, so I don't have the feeling I am wasting my time when I am contributing parts.
The downsides are:
- you still have to create many parts yourself (but parts, packages, etc are json and generating them can help a lot)
- horizon supports only newer OpenGL versions, so if your machine is old it might not run
Other than that I'd take it over all the others every day (and I do).