There is no natural scale on which to compare utility functions. [...] Unless your theory comes with a particular [interpersonal utility comparison] method, the only way of summing these utilities is to do an essentially arbitrary choice for each individual before summing. Thus standard total utilitarianism is an arbitrary sum of ill defined, non-natural objects.

This, in my opinion, is by itself a decisive argument against utilitarianism. Without these ghostly "utilities" that are supposed to be measurable and comparable interpersonally, the whole concept doesn't even being to make sense. And yet the problem is commonly ignored routinely and nonchalantly, even here, where people pride themselves on fearless and consistent reductionism.

Note that the problem is much more fundamental than just the mathematical difficulties and counter-intuitive implications of formal utilitarian theories. Even if there were no such problems, it would still be the case that the whole theory rests on an entirely imaginary foundation. Ultimately, it's a system that postulates some metaphysical entities and a categorical moral imperative stated in terms of the supposed state of these entities. Why would we privilege that over systems that postulate metaphysical entities and associated categorical imperatives of different kinds, like e.g. traditional religions?

(If someone believes that there is a way how these interpersonally comparable utilities could actually be grounded in physical reality, I'd be extremely curious to hear it.)