At the end of the day, it’s hard for me to even pinpoint what is problematic for Gentoo here: I can understand that they need to support multiple build backends as a reality of Python packaging and that’s frustrating, but per below it seems extremely unlikely that they actually need to build every single patch version of uv. Nothing about uv’s defaults or the packaging template would require that, and users would have to go out of their way to configure their build backend in that way.
(And note: this is not unique to uv! You can grief anybody who does an sdist build by pinning to a random ancient version of setuptools or any other build backend, including other ones that use native code. This is why uv, like everybody else, encourages semver ranges for build backends.)