My very weak understanding is that a lot of the C/C++ libraries heavily leverage concepts like inheritance that don't map well to Rust, and so a lot of the GPU work has been "how do we actually make this an idiomatic API?" and that has required more experimentation.

AFAIK people 100% are using other libraries for UI, but often use a macro or something to force Rust to behave in a way that those libraries expect.

I haven't read about this in literally years, but that's my recollection.