Ideal Advisor theorists attempts to define what it is for something to be of value for an agent. Because of this, their accounts needs to give an unambiguous and plausible answer in all cases.

Unambiguous? No, only an epistemic procedure of a certain sort needs to give an unambiguous answer. A metaphysical theory should give an answer that is no more precise than its subject matter. If we had a metaphysical theory of what it is to be "a heap of sand" that gave a perfectly precise yes/no answer in every case, that would be grounds for deep suspicion. A Theory of Heaps should be vague.

With theories of personal value, it's not clear that precision is a problem. I wouldn't be shocked if it turned out that "what's most beneficial to me" had a precise meaning and ranked every possible outcome unambiguously. But I would be a little surprised. (I know that Von Neumann–Morgenstern utility has been vigorously sold around here, but I haven't bought it, at least not yet.)