That's pretty much it! Although the ideas presented here do make sense (https://speakerdeck.com/vjeux/react-css-in-js) we haven't gotten there yet. Our css namespacing scheme solves the "global namespace" issue, which was the biggest issue that stood out for us.
Reasons I'd guess: not being able to use pseudo-selectors and things like :hover; familiarity for designers; easier use of legacy code; benefits of SCSS that would take work to re-implement in js. But I'm curious what your real reasons were!