This is all pretty good, but vastly over-simplified. CNC = "Computer Numerical Control". The aforementioned "5-axis" machine is similar to an old "Bridgeport" vertical mill, with motors and sensors doing the "work" instead of a machinist. In modern manufacuring of complex parts, CNC is used virtually everywhere. Those machines can do in seconds, what takes a human minutes. Imagine the labor savings in large production.
As for porting, there is a TON of misconception. IMO, the "best" machines are the Haas 7-axis jobs. Yes, upwards of a half a million to buy, program and train operators. But there's a "quirk" here. CNC machines require a "map" to follow. It's true, they are very "consistent'. The problem here is, if the human that did the orignal porting that is "mapped" made a mistake, or not a really GOOD port job, it will repeat that same bundle of errors. In one case we know of, there's a PILE of ruined iron heads where a very good "max port' job was mapped (pirated). I spoke to a CNC expert, and he said factory iron heads aren't consistent enough due to "core shift" to successfully "max port" them with the machine. Invariably, they break through bowls and runners. Another involves a popular imported aluminum small block head, that actually flowed less everywhere EXCEPT @ .600" lift, than the "as cast" version of the same head. They ruined the heads, as material was removed in the wrong places.
Pro race shops (like John Force Racing) buy semi-finished BAE heads, "roughed in" by CNC. THIS is where CNC machines offer the best service to engine builders. The crew chiefs of each team have their own "approach" to the ports and do the finish work by "hand". The Edelbrock "Pro Port" series are aimed at CNC porters.
CNC is advertised as the hot lick because it's so "consistent". When compared to an ametuer port job, they're right. But a professional porter will get the ports just as equal. Anyone that has "tried their hand" at porting knows how difficult this can be, and advertisers prey on that. They will also generally release the "peak" numbers aned not much else. Companies like Dart and Brodix, of course, have engineers "on staff" to optimize the ports. Peak numbers sound cool, but low- and mid-lift flow are MORE important to power production than peak flow.
The MOST important "negative" to remember is, machines don't "learn". Each set is the same as the previous one. So, if you want the same basic "engine" as the guy next to you, CNC heads are okay. If you want an "edge", you need to seek out a pro and follow instructions!