> If your protection is to "understand the implementation" it means buggy code.

Hilarious. Understanding the code is literally the most important thing. If you don't understand the code then you can't understand any unit tests you write either. How could you possibly claim test coverage for something you don't understand?

I suspect you primarily develop code with dynamic languages where you're reinventing type-systems day-in day-out to test your code. Personally, I try to minimise the need for unit-tests by using well-defined types and constraints. The type-system is a much better unit-tester than any human with a poor understanding of the code.