The problems arise from the fact that Mormons believe God to have once been a mortal human being who needed to reach a standard of perfection in order to be considered divine. The propositions of God being uncreated and God once being formed are mutually exclusive.
If God is not without beginning or end, then He must be subject to an additional transcendental truth (which cannot exist without an infinite mind). He must be held accountable to a standard higher than Himself. He must be under the dominion of some other gods. It would be self-contradictory to argue that God has always been eternal and unchanging while at the same time believing that He had to become a perfect deity. Yet, that is what the Mormons proclaim as truth.
The Mormon concept of God leaves us with numerous questions. Who was the first god? Who within the infinite succession of gods set the universe into motion? On what basis could there be moral absolutes? How did the Mormon god obtain omniscience in the first place (appealing to infinite regression does not get us anywhere)? If all Mormon gods are supposed to have mortal human bodies, then what about the Holy Spirit who does not have one and is considered a god?