Utilitarianism is useful in a narrow range where we have a good utility function. The problem is easiest when the different options offer the same kind of utility. For example, if every option paid out in dollars or assets with a known dollar value, then utilitarianism provides a good solution.

But when it comes to harder problems, utilitarianism runs into trouble. The solution strongly depends on the initial choice of utility function. However, we have no apparatus to reliably measure utility. You can easily use utilitarianism to extend other moral systems by trying to devise a utility function that provides the appropriate conclusions in known cases. If I do this for Confucianism, I'm being as utilitarian as someone doing it for Enlightenment teachings.