[andrewingram]

There are good designers out there who don't know HTML, CSS or JS. There are also good designers out there who know all three.

CSS-in-JS (or if not, BEM at the least) is a closer fit to the mental model of styling things in design software like Sketch and Figma. So having to use CSS in full utilising-the-cascade-and-complex-selectors mode could reasonably be seen as a bigger barrier to entry than just using something like Styled Components or Emotion. I've come across plenty of designers comfortable with CSS who felt more at home using Styled Components.

I'm not arguing that there won't be some people for whom CSS-in-JS creates barriers, but there will also be others for whom it knocks them down.