It's not a difficult problem, or even a strong knock against utilitarianism. I feel like most criticisms of utilitarianism can be countered with "that's only a problem with that poorly thought out definition X of utility that you're using - here's a better definition Y!" Worried about The Repugnant Conclusion? Maybe utility shouldn't sum linearly with the number of people you have. Maybe we should use something like Average Utilitarianism as an approach to utility instead!

I feel like the basic notion of utilitarianism should be uncontroversial. Assume there's some poorly specified but theoretically empirically measurable parameter that we can increase, that will be a Good Thing. Fumblingly try to increase it. The controversial part is which parameter is the Most Good Thing.

There are clear arguments against using "the total amount of pleasure in the universe" as a definition for utility, such as the Repugnant Conclusion and similar tiling problems, and also things like wireheading, (which some people are perfectly happy with, incidentally), and combinations of the two (rats on heroin everywhere). There are many, many alternative suggestions, each with their own benefits, each with their own fatal flaws.

That's what I see as the real problem with utilitarianism as a guide for making decisions. It's underspecified. "Maximise Utility" is the obvious part, (even if it took us several thousand years to get there) Now we need to decide which number we want to Go Up. Anyway, it's a start at least.