I think the validity of the argument is beyond dispute, I think you might have had “soundness” in mind, where the argument has the correct logical form, but there are disputes about the premises. We need to resolve this first because there is no point is discussing an invalid argument, the horse would already be dead before I hit it with my bat.
Yes, I think the argument is unsound because one of the premises is questionable, but the problem with that premise (premise 2) is so severe as to give the appearance, at least, that the argument invalid as a matter of the conclusion not following necessarily from the premises, at least with premise 2 figuring as a major aspect of the entire argument. Why do I say this? Because the assumption of premise 2, that God's being all loving has some connection to the logical impossibility of a world in which non-belief is possible is not expressed in premise 2, but simply assumed, and that assumption is neither intuitive nor logically implied, in any necessary sense, within the concept of "all-loving."
Hence, the argument, although the form is valid when expressed symbolically, appears, when expressed in natural language, to be a non sequitur because, just given the premises that:
If there is a God, he is perfectly loving.
If a perfectly loving God exists, reasonable nonbelief does not occur.
Reasonable nonbelief occurs.
One would not appear to be able, at all events, to leap from here to the conclusion that God does not exist. The conclusion appears not to have any specific logical relation to the premises because there is no clear logical or conceptual relation between belief, non-belief, and God's all-lovingness.
Admittedly, as a matter of formal logic, I'm perhaps splitting hairs, as the form is deductively valid. The problem lies in the conceptual nature of the terms used in the construction of the argument per se, not its form. The form, however, appears non-sequitur-like because premise 2 appears to create a huge logical gap that cannot be filled by the asserted conclusion.
Formal logic, however, isolated from an analysis of the concepts underlying the language forming the basis of the propositions upon which the argument is grounded, is of little usefulness in determining whether the existence of God is more or less likely, given those very underlying concepts.