I think the hard part is making a compelling UI which works in all of: up, down, left, and right reading order. Practically everyone will also be dealing with s

I think the hard part is making a compelling UI which works in all of: up, down, left, and right reading order. Practically everyone will also be dealing with something like 16:9 and 9:16 aspect ratio, so users will need to be able to transition from vertical to horizontal naturally. In my limited experience with side-panel navigation UIs I believe I would find it hard to use with my vertical displays.