With Semantic UI, you can control every single parameter, remove useless components, and you can add/override rules without messing with/modifying its files.
I tend to rank these things on a balance... of having the functionality that I'm likely to use, versus simplicity and learning curve of the functionality that's there.
So I like UIkit for side projects because it has a lot of built-in components that Bootstrap makes me find external plugins for (e.g. datepicker), and doesn't have a lot of Bootstrap crap that I never use (e.g. jumbotron, wells). It looks great, doesn't have any kind of complex "build system", and takes a few hours to learn. If I wrote a webapp that turns into the next big thing, then I'd have a real web designer redo the static stuff anyway.
How about removing SUI's jQuery dependency? [1] :)
Replacing a 100KB jQuery dependency with a 500KB React dependency I'd hardly call a solution. What happens when the next big thing hits JS land? As it stands the SUI author refuses to refactor out the jQuery dependency due to the amount of work involved.