That's the worst idea I've seen in a while. People rely on the behaviors of the GNU coreutils, this is going to break a lot of stuff. The uutils are not there yet at all.
"Differences with GNU are treated as bugs." - https://github.com/uutils/coreutils#goals
That's promising. Wouldn't it be annoying though, to have to keep things bug-for-bug compatible on a green-field project? That priority may be the first compromise, and divergence is inevitable.
(If few report edge cases and there's no fuzzing.)
[1] https://github.com/coreutils/coreutils?tab=GPL-3.0-1-ov-file