Kant criticized Anselm's argument for the existence of God by counter arguing the expression 'God exists' adds nothing to our meaning of God and so it cannot be true a God that exists is greater than a god that does not exist.
Yet, modern atheists, most frequently say such things as unicorns, dragons, fairies, ghosts, the great pumpkin, etc. do not exist. So it seems existence is a predicate according to them.
Why is existence not a true predicate with respect to Anselm's argument? What is the main difficulty with his argument besides Kant's criticism?