One weakness of the ontological argument is that its logic can let us say a lot of things exist by definition. You just have to imagine it in its most perfect form and then we can say logically that it exists by definition. This doesn’t happen so the argument is flawed and unreliable. E.g. a perfect beach, just because you can imagine something as perfect doesn’t mean that it must exist.
Another flaw is that the argument treats existence like it is a property of god, like omnipotence, but it is not. God would have to exist to have any properties as if something does not exist it cannot have properties. You must know something exists before you can know any properties it may