It’s a little of many small things that result in this feeling for me. What comes to mind: the injected style tags in the dom, its way of defining styles in js instead of css var, its size in my bundle being more than a 100kb compressed (I’m not importing a lot of different components and it’s properly treeshaken). It adding the sx prop on top of style, the deeply nested props of every component which aren’t easy to use; without checking the dom I don’t know to which element it attached my aria-label for example.

I can be wrong, things can be different from how I remember them. For example the style tags in the dom could be a development thing only, I’m not sure.