Another issue with CSS-IN-JS seems to be security - as it opens up another vector for cross-sight scripting attacks - see here for example. [1]
If I end up going with React, I'll probably avoid CSS-in-JS outside of React Native.
>You can’t use CSS in react-native apps. But you can use CSS-in-JS with styled-components. [2]
Disclaimer: met James K Nelson of reactarmory.com at #hntokyo on Thursday - otherwise unaffiliated.
[1] https://reactarmory.com/answers/how-can-i-use-css-in-js-secu...
[2] https://reactarmory.com/answers/should-i-use-css-in-js