I liked Semantic-UI a lot in terms of looks, but the React aspect of it is separate, so even though it's done by the same people as the core team, it inherently always lags behind a bit in feature completeness. I especially ran into this with transitions and animations, and migrated to Blueprint.js instead, which I was extremely happy with from a feature-completeness perspective, but which I couldn't figure out how to theme.
It's not done by the core team. It was a separate project by separate people that was brought under Semantic UI's umbrella. That's actually the major reason for the lagging behind: Semantic UI is designed for a circa 2008 jQuery website. It's a beautiful UI, but the CSS uses a horribly dated model which obligates child elements being aware of the parent elements.
End of the day, until it's rewritten with modularity in mind it will always be a drag to properly implement it in React.
I can second this. Semantic was never set up with real modularity in mind. There is tons of relational knowledge you need to have to make anything work well.
