Salam,
Imam Ali says in the first sermon of Nahj al-Balagha that God doesn't have any attributes. Imam Reza reiterates this in Sheikh Saduq's al-Tawhid. Shia Islamic scholars have interpreted this as meaning that all the traditional attributes of God (e.g. omniscience, omnipotence, etc.) are identical with God and are not distinct or "accidental properties" of God.
But this creates the problem of mercy and freedom. If mercy is an essential property of God, and not an "accidental property," then that means God must be merciful (or else He wouldn't be God anymore). This seems to contradict with the Shia Islamic traditions which say that God "chose" mercy for Himself (i.e. He could have not been merciful, but He chose to be merciful). This also makes the whole concept of thanking God meaningless, because he had to be merciful to us all the time because reality couldn't have been otherwise (he's a Necessary Being, couldn't have not existed, and couldn't have not been merciful). Why thank God for being merciful to us when things simply could not have been otherwise? It was not possible for God to have not been merciful to us, so why should we thank him for being merciful to us?
Unless there is a rational way to solve this problem, this problem demonstrates that the whole concept of a merciful, free, and "worthy of being grateful to" God is incoherent. And hence the Islamic concept of God is logically incoherent and should not be believed in.