PHILOSOPHERS conceive of the passions which harass us as vices into which men fall by their own fault, and, therefore, generally deride, bewail, or blame them, or execrate them, if they wish to seem unusually pious. And so they think they are doing something wonderful, and reaching the pinnacle of learning, when they are clever enough to bestow manifold praise on such human nature, as is nowhere to be found, and to make verbal attacks on that which, in fact, exists. For they conceive of men, not as they are, but as they themselves would like them to be. Whence it has come to pass that, instead of ethics, they have generally written satire, and that they have never conceived a theory of politics, which could be turned to use, but such as might be taken for a chimera, or might have been formed in Utopia, or in that golden age of the poets when, to be sure, there was least need of it. Accordingly, as in all sciences, which have a useful application, so especially in that of politics, theory is supposed to be at variance with practice; and no men are esteemed less fit to direct public affairs than theorists or philosophers.---Spinoza, Political Treatise.
All plans of government, which suppose great reformation in the manners of mankind, are plainly imaginary. Of this nature, are the Republic of Plato, and the Utopia of Sir Thomas More. The Oceana is the only valuable model of a commonwealth, that has yet been offered to the public.--David Hume, Idea of a Perfect Common Wealth.
To expect, indeed, that the freedom of trade should ever be entirely restored in Great Britain is as absurd as to expect that an Oceana or Utopia should ever be established in it.--Adam Smith, Wealth of Nations.
Spinoza, Hume, and Smith, all three treat More's Utopia as a species of useless and inapplicable philosophy because it (purportedly) relies on, shall we say, wishful thinking about human nature (and social reality). In doing so they echo a Machiavellian trope (who may not have known Utopia, and is probably referring to Plato's Republic):
it being my intention to write a thing which shall be useful to him who apprehends it, it appears to me more appropriate to follow up the real truth of a matter than the imagination of it; for many have pictured republics and principalities which in fact have never been known or seen, because how one lives is so far distant from how one ought to live, that he who neglects what is done for what ought to be done, sooner effects his ruin than his preservation. Machiavelli, The Prince, Chapter 15.
Undoubtedly some readers would not mind having their vain efforts being lumped together with Plato's Republic.
What's notable about Spinoza's criticism of Utopia is that he treats it as the exemplar of what philosophers do. (Presumably, Spinoza thinks of his own work as true philosophy, and the to be rejected style of philosophy as false [in the manner of philosophical prophecy].) And even in Hume's criticism, it is still mentioned alongside that classical, paradigmatic text (The Republic). It's hard to imagine recent philosophical authors to accord Utopia such high status--even as the to-be-rejected other.* Undoubtedly, this has something to do with the fact that for recent political philosophers, our story starts with Hobbes and his way of framing matters has endured (so far).
As I have noted before (here and, especially, here), Utopia prefigures quite a bit of Spinozism. More important, the Machiavellian three (Spinoza, Hume, Smith) treat the Utopia as if it does not contain part 1, where the utility of a realistic philosophy in less-than-perfect-circumstances is heavily debated by the characters. In fact, we may say that the Machiavellian three ignore not just More's explicit distinction between the academic and civilized philosophy (and the third prophetic kind, which I claim More adopts), but unfairly typecast Utopia as (in his terminology) a species of naive academic philosopher (despite the self-presentation of More in Utopia as a man of practical affairs, that is, a lawyer and diplomat). Undoubtedly, More's work exhibits limitations, but it is not the one that is commonly attributed to him.
Unlike Machiavelli, Spinoza and Hume (Le Bon David) have managed to acquire a reputation of almost saintly philosophical integrity and nobility. One does not reduce their genuine greatness by noting their inability to give More his due. I mention this not to score-keep the history of philosophy, but to remind ourselves of a homely truth that even when it is said by the highest authorities that some other philosophy is useless, it need not be so.
*In New Atlantis, it is treated as a book that is seriously studied (and improved upon) in Bensalem.