I will disagree with the above statement simply because it is possible to achieve excellent cylinder head performance with hand porting. The Key with hand porting is only remove the minimum amount of material to get the job done. The big disadvantage of hand porting is that it is an incredibly time consuming operation that will result in many hours of labor The main advantage of CNC type porting is a reduction in porting time especially in a production environment so this is why shops use CNC equipment to port heads.This kind of performance is not achieved by duplicating YouTube porting video clips. Remember when you hurt the low lift flow you profoundly injure all but wide open throttle high rpm performance. You can turn an OK performer into an unhappy and unpleasant to drive engine / car.
Not my work, but here is home port job that addresses the bowls that most critical area for flow in the 4.6 head
Of course if you hog out the ports, velocity and therefore performance will suffer.
In this exhaust port, the gasket area has ben marked so it is not removed. the goal is simply to blend the walls and knife edge the divider.
Even Accufab Racing will finish port their heads by hand. I would suspect that the initial CNC port is done to speed up the process.
"Accufab does its own porting, starting with a CNC program and finishing by hand. The intake ports are not radically changed. The floor of the port is simply cleaned, while the roof and bowl are opened around the backside of the bowl. This makes more of an oval shape relatively deep in the intake port to keep the cross-sectional area the same throughout the port."