I agree with this in the fact that Darkseid may be too deep and powerful to be adapted as a logical villain to fight Superman. He is as powerful as the Man of Steel to a human, there could never be a physical confrontation between the two. Every aggressive action against Earth would have to take place in the forms of his minions or military, and little developing the actual villain. Plus, with Darkseid, the mythology of New Gods needs be explained fully, something too big and irrelevant with the themes of Superman.
Mongul is much closer to being mortal and Supes can actually give him a challenge in a effectively workable storyline.