I like that it's modular, easy to use in combination with other libraries such as react-bootstrap or material design components (like buttons).
It also has enough flexibility so you can customize your own styles without using any CSS framework if required by some particular project requirement which might not be available out of box from semantic ui library itself! There are few things i don't really understand fully yet but nothing much too big deal either since there was no documentation at all when we were first trying this product - just go through their website examples they provide :) We used them mostly together wth BootStrap 4 where everything looks great right away after installing semantui module via yarn add --dev @semantics/react!