To be fair, I believe the test suite has improved over the years, but adding new features requires meticulous caution to make sure you don't miss any edge cases

To be fair, I believe the test suite has improved over the years, but adding new features requires meticulous caution to make sure you don't miss any edge cases, at least on the functional side. But then you occasionally get bugs like https://github.com/openzfs/zfs/issues/15453 https://github.com/openzfs/zfs/issues/15453 which can't easily be caught by test suites.