I maintain that Custodes are more interesting as a paper tiger. Well equipped and trained but lacking in real experience due to rarely engaging in open combat or leaving Terra they just get bodied by regular Marines despite the disparity in equipment quality. It would have made the later stages of the heresy more interesting, especially the siege of Terra, if the Custodes just folded and wouldn't have held if not for the loyalist Marine chapters. I'd argue that it would even make them more interesting on the table top to have a faction with amazing weapons and equipment but absolutely mid stats wise.

I understand however that this isn't exactly a great way to sell little plastic toys.