d-a, the problem at Melbourne was not the use of flex but the incorrect weight loaded into the FMC. The problem was rectified by judicious application of power which was almost too late but did finally get the beastie off the ground after causing some havoc with runway lights and aerials.
As for QF1, there were many other problems that led to that little overland excursion other than merely the use of reverse thrust or not. To that airlines credit they learned very quickly.