Variants are not the same as having an API to override styles of inner components. You can use variants in Material UI too (which btw is inspired by styled-system and prior art): https://material-ui.com/system/basics/#variants

Variants are defined by the implementor of the component in question. What I am talking about is overriding styles by the user of the component. One way to do it is through the ThemeProvider. But that does so globally. If I want to override a style for a particular component only in one place (and the nested components within it) then I will have to use CSS nested selectors or a nested ThemeProvider (which is hacky and ends up slowing down the app depending on how it is implemented). Then I am tightly coupling the implementation with the presentation. There is no API as such to change the nested component styles directly. That is the area where I feel Material UI does better. It has standardized how inner components are exposed to the World for styling. That standardization should happen at the library level instead of at application level. So whenever you import the library you know you have that option available to you too.