That he makes assumptions is no point against him; the question is do those assumptions hold.
To support the first one: the popularity and success of the fallacy of appealing to authority, Milgram's comments on his experiment, the "hole-shaped God" theory (well supported).
For the second one: First, it's not entirely clear we do understand expected utility maximisation. Certainly, I know of no-one who acts as though they are maximising their expected utility. Second, to the extent that we do understand it, I would draw the metaphor of a Turing tarpit - I would say that we understand it only in the sense that we can hack together a bunch of neural processes that do other things, in such a way that they produce the words "expected utility maximisation" and the concept "act to get the most of what you really want". This is still an understanding, of course, but in no way do we have machinery for that purpose like how we have machinery for orders from authority / deontological injunctions.
"Expected utility maximisation" is, by definition what actually represents our best outcome. To the extent that it doesn't, it is a failure of our ability to grasp and apply the concept, not a failure in the concept itself.
As for the third, and for your claim of debatable: Yes, you could debate it. You would have to stand on some very wide definitions of entirely and different, and you'd lose the debate. For example: speaking aloud to an AI and speaking aloud to a human are entirely different tasks. Not to mention that conveying a concept to a human carries no instructions; programming concepts into an AI is all instructions. Another entire difference.
"Expected utility maximisation" is, by definition what actually represents our best outcome.
No, it's based on certain axioms that are not unbreakable in strange contexts, which in turn assume a certain conceptual framework (where you can, say, enumerate possibilities in a certain way).
Barring a major collapse of human civilization (due to nuclear war, asteroid impact, etc.), many experts expect the intelligence explosion Singularity to occur within 50-200 years.
That fact means that many philosophical problems, about which philosophers have argued for millennia, are suddenly very urgent.
Those concerned with the fate of the galaxy must say to the philosophers: "Too slow! Stop screwing around with transcendental ethics and qualitative epistemologies! Start thinking with the precision of an AI researcher and solve these problems!"
If a near-future AI will determine the fate of the galaxy, we need to figure out what values we ought to give it. Should it ensure animal welfare? Is growing the human population a good thing?
But those are questions of applied ethics. More fundamental are the questions about which normative ethics to give the AI: How would the AI decide if animal welfare or large human populations were good? What rulebook should it use to answer novel moral questions that arise in the future?
But even more fundamental are the questions of meta-ethics. What do moral terms mean? Do moral facts exist? What justifies one normative rulebook over the other?
The answers to these meta-ethical questions will determine the answers to the questions of normative ethics, which, if we are successful in planning the intelligence explosion, will determine the fate of the galaxy.
Eliezer Yudkowsky has put forward one meta-ethical theory, which informs his plan for Friendly AI: Coherent Extrapolated Volition. But what if that meta-ethical theory is wrong? The galaxy is at stake.
Princeton philosopher Richard Chappell worries about how Eliezer's meta-ethical theory depends on rigid designation, which in this context may amount to something like a semantic "trick." Previously and independently, an Oxford philosopher expressed the same worry to me in private.
Eliezer's theory also employs something like the method of reflective equilibrium, about which there are many grave concerns from Eliezer's fellow naturalists, including Richard Brandt, Richard Hare, Robert Cummins, Stephen Stich, and others.
My point is not to beat up on Eliezer's meta-ethical views. I don't even know if they're wrong. Eliezer is wickedly smart. He is highly trained in the skills of overcoming biases and properly proportioning beliefs to the evidence. He thinks with the precision of an AI researcher. In my opinion, that gives him large advantages over most philosophers. When Eliezer states and defends a particular view, I take that as significant Bayesian evidence for reforming my beliefs.
Rather, my point is that we need lots of smart people working on these meta-ethical questions. We need to solve these problems, and quickly. The universe will not wait for the pace of traditional philosophy to catch up.