The big difference is with run-time styling changes when communicating with an API. We ship an application as a whitelabel where our customers define their theme through Contentful. With CSS-in-JS, converting the entire app's theme based upon XHR is trivial.

Easily done in Sass env but requires tooling/work: - Automatically shipping the smallest styles per page/component. - Automatic autoprefixing - Hashed selectors (avoidance of conflicts) - Tree-shaking (no unused styles)