What is the preferred

Input

usage style: controlled or uncontrolled? How can I decide?

I think that controlled is useful in most simple cases and uncontrolled for complicated scenarios.

With controlled TextInput fixed in beta4 (#1169), what are the remaining use cases for uncontrolled inputs? I am working on an MDC text field for Compose, which needs to synchronize the state of a floating label and an outline notch with its input. This seems way easier when not exposing the uncontrolled API. I wonder whether there is any downside when only the controlled API is available for component users.

Reasons to have both are same as in react : https://reactjs.org/docs/uncontrolled-components.html

Since an uncontrolled component keeps the source of truth in the DOM, it is sometimes easier to integrate React and non-React code when using uncontrolled components. It can also be slightly less code if you want to be quick and dirty. Otherwise, you should usually use controlled components.

If you're implementing a new component you can make "controlled" only for the beginning until you see a need to have it uncontrolled.

So at least in my MDC use case, it's just the opposite: Easier to integrate in controlled mode. I'll be following that path until YAGNI evaluates to

false

.