I really like material-ui but am constantly bummed about their use of inline styles.
Each component has multiple style props, lest you resort to "div > div > div:nth-child(2)" in your own CSS.
Each "style" prop now has to be diffed by React too.
Doing things like hover on mouse/touch events seems like it will never be as efficient as using ":hover".
And when doing universal rendering, the amount of markup you send is huge. And you have to disable the browser-prefixing of style props.
Having said all that, the material-ui library is still great.