> If you have an unit test on private members, then you can't change the implementation safely without breaking tests.

This makes absolutely no sense

You change the private implementation, you change the unit test. It's that simple

Then you keep your API the same so that external users don't break.

It seems to be from the same people that like to whine about "missing tests and lack of coverage" quite funnily. It seems they like to nitpick and idolize tests instead of shipping