I am attracted to utilitarianism, but also find some of the possible implications off-putting. But there are also some objections I have from first principles.
One objection is that any numbers we use in practice just have to be made up. (This objection might be especially serious if we take animals into account, which I think we should.) So maybe utalitarianism is the “correct” theory but if I don’t have access to the correct utilities it is not clear whether I should use some made up numbers to do the expected utility calculations. One might compare with theorems saying that individual rational choice is equivalent to maximizing a von Neumann-Morgenstern utility function. Yet very few people, even economists, try to do that in practice and it is not clear that people would be less irrational in practice if they tried to do calculations with their expected utility in various circumstances.
A second theoretical objection I have is that if we suppose there is any chance that humanity, or sentient life, will survive forever, then the universe will contain infinite amounts of pain and pleasure, all calculations become divergent, and the theory gives no guide at all. You might object that this is impossible with current scientific theories, but the conclusion goes through no matter how small the probability is. Surely there is a 1/Ackerman(1000) chance that our current understanding of physics is wrong?