The problem I found with petrifex is not that is much stronger than the other legions (it is), but that the other legions feel... wrong, and it isn't by comparison, petrifex is that strong because the battle trait and command ability, and the reason the other legions feel wrong are the command traits and artifacts. All of our generic heroes are support, even the Liege-Kavalos is taken for the RD points and CA, so you don't want combat focused command traits or artifacts, and the only ones that aren't combat focused are the Petrifex and Mortis Praetorians command traits and the Petrifex Artifact (and the Stalliarch Lords if you don't take an artifact in a Liege-Kavalos).
And even if we ignore that except Petrifex and Crematorians the others have extra problems in how they feel:
Mortis Praetorians have the feel that you have to take named characters
Null Myriad have the feelthat you will completely shut yout opponent (against heavy magic list) or that you will be playing without legion
Stalliarch Lords feel cavalry oriented, but there is only one cavalry unit, and the batallion already gives that CA so the utility of Stalliarch is for... Nagash?
Ivory Host is just bad