Besides the early fallout of switching to Rust Coreutils on Ubuntu 25.10 causing some breakage, a more pressing issue has been discovered: Ubuntu 25.10’s unattended upgrades functionality for automatic security updates is currently broken due to a Rust Coreutils bug.
Earlier this month it was reported that the date -r command can report the wrong date on Ubuntu 25.10 due to a Rust Coreutils difference compared to GNU Coreutils. It was noted that this could cause issues for backup scripts and other software relying on the “date -r” output and behavior being the same as GNU Coreutils
At least from test coverage point of view, I think they aim to have the same tests to pass as the original tools. Not all pass right now, so incompatibility is expected at some point, somewhere.