Without independence or integrity then AP and FD could suffer similar error, e.g common sensor or input failure.Monitoring must involve independently comparing the autopilot or FD against the aircraft response - the actual flight path, the real world, not as expected or computed.
There are three ADRs and yet unreliable speed happens and that's why the pilot is there to take over. I have said it a few times before whether the AP is on or off pilot's scan or monitoring does not change. With AP on AP makes flight path changes with AP off pilot does it himself. In modern Aircraft pilot also has to know the automation well. Nobody denies that pilot must possess basic flying skills but you cannot turn automation into a villain. Better redundancy and automation is also reducing the requirement of very high standard of basic manual skills.