I've recently done a project with Semantic-UI, and there are still a lot of gocha's with specific nestings you'll have to follow and some ui blocks that don't mix wel, but the worst thing for me are the modals. If you have more than one modal, you will get both modals being triggered at some times, sometimes, you'll get the fade in/out animation 2 or 3 times, etc.

It's still a young project, and imho one of the best looking ones, but there are still a lot of quirks to work out.