As I understand it, there could be two reasons:
First, emotional...since it is a replacement part made and sold after the original timespan of Cobra production...hence, less "correct"
Second, technical... since the blocks were made after complete engine production of the 427 was shutdown, there is a higher likelihood of "defective" attributes on service blocks, like core shift when the block was cast.
As stated above...we need to hear from the real FE experts on this...(George?) ...sometimes I tend to swim in hearsay.
