The ontological arguments of God are many, the weaknesses they suffer can be found in this reference.
My goal here is to focus on Spinoza’s ontological argument. Spinoza's argument is different from the others in that it does not fall back to many ambiguities (like the absolute perfection in goodness), Spinoza’s God is the “totality of what is”, this restriction could provide more basis for reasoning as all attributes of God are positive ones (by positive is meant which is, as opposed to which is not). This ends up equating God to the Universe, which avoids all the weaknesses of the other ontological arguments.
If Spinoza was content with equating God to the Universe, then all would’ve been fine.
However, Spinoza’s pushes for a definition (which is not to be proven), where he states :
“By God (Deus) I understand a being absolutely infinite, that is, a substance consisting of infinite attributes, each of which expresses eternal and infinite essence.”
Since all we can attest to is Extension (and Mind if we agree to a separation of Mind from Extension) the definition is weakened. Obviously, Spinoza’s reasoning is that restricting the number of God’s attributes is a “contradiction” with his absoluteness, the underlying motive is conserving the necessity of “perfection”. But this seems at odd with the general trend of his philosophy (naturalism).
My question is why did Spinoza choose to include this definition when he clearly was so close to a naturalistic understanding of God as a pure “totality of all existence”? The gist of his work would’ve been more consistent with this rephrasing of said definition: “By God I understand a being absolutely maximal in existence, that is a substance consisting of all possible attributes, each of which are essential”.