CNC porting is only as good as the information fed into the computer, and even then it still falls short of the hand ported port that was digitized for the CNC program. I would say that on a head that has symetrical ports you could get away with a fairly good CNC ported head. But on a big or small chrysler where you have ports with opposite bias, there's only so far a CNC machine can go IF there's core shift. This is where a good head porter comes in. I like to start max effort port jobs with untouched heads. It gives me an idea of how thick the material is. If I start on a CNC'd head I have no idea how much material is left in the port and whether or not there's any core shift or not.
For something that I just want a quick port job on and consistency, I say go CNC. But if you want the most out of a set of heads, you need a good head porter. As for the debate of hearing when the material gets thin, I have experienced this on several occasions.
If you want the best CNC ported heads out there, go talk to Chapman. They are an example of CNC porting at it's best.