Denys Turner ( Eros and Allegory: Medieval Exegesis of the Song of Songs (Cistercian Studies Series) 
First, Platonism has difficulty explaining how anything can exist other than God: “For the platonising Christian, God is above all else unity, a oneness beyond differentiation. If anything at all other than God exists, then God’s absolute oneness appears threatened. For if there is anything other than God it would seem to follow that there is something in God which is his otherness than creation, some character of being differentiated. But if there is anything from which God has to be differentiated, then it further seems to follow that God is a being whose nature is, in part, that by virtue of which he is different from anything else. It seems to follow, in short, that God is but one kind of being among a multiplicity of kinds of being, a being among others.” Since God is not just another being among beings, it seems that “there cannot be anything other than God, nothing can exist ad extra , if the absolute unity and undifferentiation of God is to be preserved.”
But there’s an equal problem with trying to make room for God as a cause of anything else: “How can that which is absolutely ‘one’ cause multiplicity without itself falling from oneness? The mere existence of anything other than God would appear to set God within a complex of external differentiations. But worse, for God to cause anything other than himself would prima facie be to suppose differentiation within the very being of the godhead itself. And there cannot be differentiation within that which is the cause of all, for internal differentiation and multiplicity is a characteristic precisely of creatures. Consequently, God cannot cause multiplicity to exist without himself becoming multiple. But for God to become multiple is for God to cease to be God. Therefore, God cannot cause anything other than himself to exist.”
The only way out for the Platonist is a differentiated God. The only way out is Trinity.