Evidence suggests that one of the worst persons for assessing level of fatigue is the person concerned. While in some cases it is clear, often it is not. Surely this is going to push more people to work harder and run on the ragged edge more often. Squeezed by the employer if they are perceived to be calling a fatigue issue and then completely juiced by the system if they don't and proceed to screw up in any way.
Onus needs to be placed on the operator to make sure that they are not rostering/requiring more than what should be reasonably expected. CAO48 was a rough tool but at least it placed clear limits on what some operators could attempt.