At first, I was a bit skeptical, because there have been numerous attempts to attack Benatar's asymmetry, and all of them have failed miserably (even if we're charitable). But to my surprise, I found this paper to be extremely compelling, and I realized that many of the points that Cabrera brought up had been subconsciously mulling around in the back of my mind but I hadn't been able to piece the problems together in the way that Cabrera has done. It truly is unfortunate that non-English-speaking philosophers tend to be passed over, as Cabrera is, in my opinion, a top-notch, original thinker.
There are many points that Cabrera brings up. The most striking and important is that he claims that there is no asymmetry at all.
A refresh of Benatar's argument: pleasure for the existing is good, pain for the existing is bad, absence of pleasure for the non-existent is not-bad, and absence of pain for the non-existent is good. Therefore, it is claimed, birth is always a harm.
Cabrera finds fault, though, in Benatar's apparent use (or abuse) of counterfactuals (if...then). Benatar applies the counterfactual to the potential, non-existent person in the case of the absence of pain, but in the absence of pleasure, he does not apply the counterfactual. Instead, in the case of the absence of pleasure, he uses an "empty" placeholder for a potential, non-existent person.
It is this misuse of counterfactuals that makes Cabrera argue that there is actually no asymmetry at all, and that if Benatar is going to use counterfactuals for the absence of pain, he must use counterfactuals for the absence of pleasure as well. Therefore, the absence of pain is good, while the absence of pleasure is bad, and therefore, there is a symmetry instead of an asymmetry. If the absence of pain is good, then the absence of pleasure must be bad. If the absence of pleasure is not-bad, then the absence of pain must be not-good.
There are other points brought up in the paper that are also interesting. It should be said, however, that the author himself is in fact an antinatalist and a rather extreme pessimist (re: negative ethics and the critique of affirmative ethics), who was operating under such a guise for around thirty years prior to the publication of Benatar's controversial book.
To my knowledge, Benatar has not responded to Cabrera's criticisms. I believe that Cabrera is entirely correct in his critique of the abuse of counterfactuals. The Benatarian asymmetry leads to absurd conclusions, in which even if the life of an individual could be envisioned and known to be "good" and "worth living", it would be immoral to conceive them because they might stub their toe. As a quick heuristic, the Benatarian asymmetry may be useful, but as a rigorous model I think it fails to hold.