Is it a harm or benefit to come into existence? Is it a harm or benefit to cease to exist? Is it a harm or benefit to cease to exist earlier rather than later?
I shall suggest that there is no answer to these questions, as pain and pleasure are incommensurable, the intensities of pain and pleasure are immeasurable, and the pain and pleasure experiences are quite often incomparable.
Moral and axiological anti-realism is also a reason of the impossibility of population ethics.
>>(2) Impossibility of comparing existence and non-existence
146 My analysis of the dissenting judgments in Cattanach shows the legitimacy of approaching a novel tort problem by considering whether courts can make a rational and just comparison between the plaintiff’s condition affected and unaffected by the impact of the defendant’s conduct. If the two divergent lines cannot be depicted then the law has no framework for attempting the “costing” exercise. Impossibility, not difficulty, is the touchstone (Cattanach at  per Hayne J).
147 This basal legal principle was invoked in the present context in several of the American cases and in McKay, the English decision cited with apparent approval by Gleeson CJ and Heydon J in Cattanach. On this analysis, any attempt to compare existence with non-existence is regarded as entering the realms of theology, metaphysics or pure speculation and thus, so the argument goes, beyond the ken of the law<<
-Harriton (by her tutor) v Stephens; Waller (by his tutor) v James & Anor; Waller (by his tutor) v Hoolahan  NSWCA 93
Improvability of Benatar’s Asymmetry
Benatar’s asymmetry is a philosophical position which considers that experience of pain is a harm of coming into existence, while experience of pleasure is not a benefit of coming into existence.
Criticism of Benatar’s asymmetry was raised by Julio Cabrera.
Also, it is difficult to see why coming into a eternal (immortal) life of a complete bliss cannot be a benefit.
My argument against Benatar’s Asymmetry
Life A) Life with 10 units of pleasure and 10 units of pain
Life B) Life with 0 unit of pleasure and 10 units of pain
It is certainly the case life A is a preferable life to come into existence to life B. Yet, Benatar’s asymmetry judges two lives of same harm of coming into existence.
Irrefutability of Procreative and Mortal Symmetry
Procreative symmetry is a philosophical position which considers that experience of pain is a harm of coming into existence, and experience of pleasure is a benefit of coming into existence.
Mortal symmetry is a philosophical position which considers that pain is a harm of coming into existence, and pleasure is a benefit of coming into existence.
It is impossible to refute the procreative and mortal symmetry, and both are quite universally accepted position among philosophers and lay people.
Many, if not most, (relatively happy) lay people think that they were benefited by being brought into existence, and appreciate their parents for bringing them into existence.
Incommensurability of pain and pleasure
Pain and pleasure are incommensurable. The difference of pain and pleasure is of kind, not degree. In other words, presence of so-called one unit of pain is not an axiological equivalent of presence of so-called minus one unit of pleasure.
It follows from this people have different preferences in pain-pleasure trade-off. While people who are more pleasure-seeking than pain-avoiding indulge in activities such as binge drinking (which brings pleasure of intoxication with pain of hangover, albeit with some temporal difference), people who are more pain-avoiding than pleasure-seeking refrain from such activities.
Although there will be average or median of the pain-pleasure preference, it is difficult to see the average preference is of metaphysical significance.
Immeasurability of pain and pleasure intensities
The physical value of pain or pleasure inducing stimuli or intervention are certainly measurable.
For example, the voltage, current, energy and energy per time of an electric shock is measurable. A kinetic energy of a projectile is measurable. An amount of alcohol is measurable.
However, the so-called amount of pain and pleasure those stimuli or interventions cause to the consciousness is impossible to objectively measure, at least with current technology and science.
If there are two projectile that hits the same body parts with same vector (same direction and speed):
Projectile A) Sphere with 1 cm diameter, Density 1 g/cm^3
Projectile B) Sphere with 1 cm diameter, Density 2 g/cm^3
While projectile A has two times as kinetic energy as projectile B, how can we say projectile B cause two times pain as projectile A?
Of course, there will be preferences, when people are given choice of getting hit by either X times with projectile A or Y times with projectile B, there will be a ratio of X:Y which a person will be indifferent. However, it is difficult to see the average or median ratio of indifference of people is of metaphysical significance.
Similar is the case for pleasure. Provided that there will be no hangover, how can we say 2 mL of alcohol bring about two times as pleasure as 1 mL of alcohol?
Many people prefer short but intense suffering to less intense but protracted suffering. Also many people prefer longer but less intense suffering to shorter but very intense suffering.
For example, some convicts, when given a choice between a caning and an incarceration, will choose the former, while some will choose the latter.
Indeed, many of corporal punishment in schools in Southern United States are practiced with an option to take detention instead. Some students choose short but intense suffering of paddling over less intense but protracted suffering of detention, while some students choose long but less intense suffering of detention over intense pain of paddling.
Incomparability of different pain and pleasure experiences
For example, the badness of mental suffering and physical pain are incomparable in its intensity. Although there will be preferences, it is difficult to see the average or median ratio of indifference of people is of metaphysical significance.