It's one thing to copy and acknowledge the QNH, quite another to set the altimeter(s) correctly. If errors occur this implies that the pilots concerned are not checking the altitude is correct against apron elevation having set the subscale. Even if you do set the QNH correctly it could be the indicated altitude that's incorrect if the altimeter/adc etc is giving the wrong information.
You can pass and readback the QNH as many times as you like but the pilot still has to set the subscale correctly.