Utilitarianism is fundamentally incompatible with value complexity.

To me it seems that if you value multiple things, let's call them A, B, C, you could construct a function

It sounds like you might be confusing utilitarianism with utility functions (a common mistake on LW). While utilitarianism always involves a utility function, not all utility functions are utilitarian.

Even if you have complex value, you may find that in current situation the best way to increase total outcome is to focus on one of these values.

Yes, that's always theoretically possible. In real life, however, humans are subject to value drift, and have to "practice" their values, lest they lose them.

One person proposes a new food program. Another person proposes making another movie (of which you already have a few dozens). As a mayor, you choose to spend the tax money on the former. The latter guy accuses you of not understanding the complexity of values.

That doesn't sound like the latter guy's true rejection. It sounds like he really means to accuse the mayor of undervaluing movies specifically. (After all, if the mayor had made the opposite choice, why couldn't the food program guy equally well accuse the mayor of not understanding the complexity of value?)