I've had several abused CX500 engines with badly worn cam followers and cam-shafts.In all cases it was Pilot error by POs
The main cause was incorrectly installed cam followers orientated the wrong way round.There is an A and B type and they must be fitted with the cups at the correct angle to receive the ends of the Push rods.Wrong way round and they apply incorrect pressure on the Cam lobes and the cam follower faces.
In this picture the two on the left would be correct and the two on the right would be wrong and cause premature component failure
I also found the oil orifices reversed and or blocked on some of these engines which will also cause top-end oil starvation.
I don't think there are any inherent faults with the design of the 500s or 650s.