react-aria is awesome. You structure your DOM and css however you want, and react-aria provides hooks that return props to spread onto your elements to make them come alive. These same hooks also give you visibility into and control of the component state, so you can also easily blend in your own custom logic for the component. It is about as flexible as a headless ui library can get. But it's also lower level and so harder to use than, say, radix primitives.

I work on Plasmic (plasmic.app), a visual builder for React. With react-aria, we're able to let our users build, say, a Select component visually however they want, and then we just use the hooks to spread the right props to the right elements, and It Just Works. See https://youtu.be/mPHS2zg2a8Y

Headless UI is an exciting space. There's only so far you can go with re-skinning material ui or antd, and no one should be building their own from scratch. These headless libraries give you all the interactivity and accessibility you need for your components, but still give you total freedom over how they look. It is the way!