The main difference between a utility function based approach is that there is no concept of "sufficient effort". Every action gets an (expected) utility attached to it. Sending £10 to an efficient charity is X utilitons above not doing so; but selling everything you own to donate to the charity is (normally) even higher.

So I think the criticism is accurate, in that humans almost never achieve perfection following utility; there's always room for more effort, and there's no distinction between actions that are "allowed" versus "required" (as other ethical systems sometimes have). So for certain types of mind (perfectionists or those with a high desire for closure), a utility function based morality demands ever more: they can never satisfy the requirements of their morality. Those who are content with "doing much better" rather than "doing the absolute best" won't find it so cripling.

Or, put more simply, for a utility function based approach, no one is going to (figuratively) hand you a medal and say "well done, you've done enough". Some people see this as being equivalent with "you're obliged to do the maximum."