I'd argue this kind of notion is comparable to ideas like "benevolent dictators" or surveillance supposed used only for people's protection, in that, in theory if you had someone who both had access to sufficiently perfect information and was incorruptible with everyone's best interests at heart running it- and their successor and all later ones also fit that description, then logically it ought to be beneficial. The problem being, of course, that in the real world that never happens.

People don't have a perfect understanding of medicine and it's difficult to tell if a deviation from one's personal norms is beneficial or harmful or neutral, especially if you cannot ask the person involved for their feelings on it, since you're determining if they should even be born or not. It's not necessarily even an objective standard, one person might dislike living with a trait that another person might consider valuable. People are innately biased, even when they are aware if this and try not to be, and as such it'd be much easier to see the negative side to any trait one finds aesthetically unpleasing, even if that person is determined to ignore aesthetics.

Now, there's a counterpoint that can be made that, while we may be imperfect at determining it, there are still quite a few conditions that are unambiguously harmful and undesired by effectively everyone, and that some of these things are genetic and therefore heritable. The entire concept of "genetic diseases" exists for a reason after all, and therefore one could argue that we could just test for these, and just limit eugenics to keeping people with those specific genes from reproducing, and that maybe then the potential benefits might outweigh the harm.

But even then, I don't think it can be justified. The sorts of technologies that would let us determine if someone is a carrier for genetic disease also open up potential pathways for those people have children if they want them without passing on that specific gene, like ivf with an embyro chosen that didn't inherit the disease, or potentially genetic manipulation of a zygote or gamete cell to change the specific gene responsible for an illness (that one might not be entirely ready now, but given how long human generations are, it certainly should be by the time old school selective breeding would make a difference even with hypothetically perfect intentions).

Those ideas have their ethical troubles too of course, before someone brings up "Gattaca" or the like, but my point is that given technology exists, or can be reasonably be expected to be available before enough generations have passed for selective breeding to do much, that offer any theoretical benefit a eugenics program might have, without having to forbid people from having children (which inflicts significant distress and therefore harm on those people by taking away their agency), there is no longer any point to it even if you could, somehow, prevent it from being used to further racism and the like (and again, I'm very skeptical of that being doable anyway). As a technology, it's already obsolete, on top of it's terrible track-record, and is simply not worth considering.