it's a stretch to "executing a script with a build user" or "from a validated distro immutable package" to "allowing something to download evergreen code and install files everywhere on the system".
uv installing deps is hardly more risky.
Scanning for external dependencies is common but not so much internal private libraries.
I've used Tiger/Saint/Satan/COPS in the distant past. But I think they're somewhat obsoleted by modern packaging and security like apparmor and selinux, not to mention docker and similar isolators.
uv executes http://somemirror.com/some-version
most people like their distro to vet these things. uv et all had a reason when Python2 and 3 were a mess. i think that time is way behind us. pip is mostly to install libraries, and even that is mostly already done by the distros.