Could you give some examples of the complexities you're tired of?
For one, we had to rewrite the app 3 times because the library switched from Less, to 2 different css-in-js solutions across 3 different major versions.
Then we switched to server side rendering and suffered some weird glitches. Turns out, when using SSR, styles get rendered twice: once on the server and once in the browser. So you have to write a custom logic to remove the styles that are rendered on the server.
What I'm trying to say is that React was invented at Facebook to solve problems they were having but the industry was too quick to adopt it as a solution for their own problems. Facebook can afford to rewrite a piece of software 100 times because of their unlimited budget. They can also deal with all the bs that comes with frontend frameworks because of their ability to throw an unlimited amount of engineers at a problem.
Small company with 10 engineers? Just use something appropriate for problems you're trying to solve at your budget.
https://mui.com/system/styles/basics/ It looks like MUI had some decent intentions in their changes.
https://mui.com/material-ui/guides/server-rendering/#mui-on-... It also looks like they called out how to address the SSR issue (maybe they recently added these docs?).
I kind of accept this type of friction as a part of libraries changing (As long as the upgrade paths aren't insane).
React itself isn't really the problem because there are some valid use cases for it. However, it's currently being thrown at every web development problem even when there are better and more stable solutions.
But if I had to use React today, I'd stay away from MUI for sure since they constantly demonstrated a lack of maturity.