a CNC is just a machine, it doesnt really have an effect on the quality of the final product.
quality of final product is a result of design and quality of materials used. CNC's are used on both casted and forged materials.
the benifit of a CNC is the speed with which you can make multiple parts, and speed from design to finished product.
also some items of extremely high tolerances need a computer oprerated machine to be made.
when it comes to bike parts saying something is cnc'd probably doesnt really mean anything. i think its just an advertising gimmick. whats important is the quality of the material, the design and the way in which it was machined.
you could put a piece of balsa wood in a CNC and it would cut out a crank for you, or you could carve one by hand, it might look good, but you wouldnt want to put it on your bike.