Well, makes UI more optimized and ease of use. aswell supports all platforms (besides Education Edition) and proper support for controller.
This UI isn't a client, so this basically acts like your regular texture packs. this also does not modify or give you any advantages whatsoever.
All in-game UI's are compatible. Although it does not compatible with any UI packs that modifies pre-game UI.
Collapse-UI offers some and customizability with the theme and layout.
You can change about how would UI look, e.g Red accents, backgrounds, etc. Look for "Configuration" section in Installation page.