[Spoilers for the third book]
My previous post was about inconsistencies in the books' worldbuilding. Working those out and arguing about them, treating the fictional world as if it were real, is fun but not very important. In my view there is only one thing significantly wrong with what are, in most ways, very good books.
A major theme of the series is the undesirable and unattractive consequences of large inequalities of wealth, power and status, as illustrated by the interaction between Enclavers and Indies in the school. That is, in my view, a real issue. One of the many things I admire about my father is that even after he was very famous he treated other people as equals, had, in the British sense, no side. Some high status people act that way but many do not.
In the first two books, the inequality shown can be reasonably identified with its real world equivalent. In the third book it cannot. One can argue that rich people should treat other people better and share more of their wealth, as one can argue that the Enclavers should have taken less advantage of the Indies. But one cannot argue, at least not plausibly, that every billionaire got that way by torturing someone to death, which would be the equivalent of what we learn about the creation of enclaves in the final volume.
Someone might respond that every billionaire could have used his wealth to save the life of multiple starving children in Africa, that failing to do that is the equivalent of crushing Liu and loosing a mawmouth to consume random foreign mages. But it is not true — or at least it does not fit the moral intuitions of most people including, pretty clearly, Novik, who has already shown us enclaves failing to keep Indies alive, both in the Scholomance and the outside world.
There is a sharp distinction in our moral intuitions between the consequences of action and of inaction. If failing to save someone's life is the moral equivalent of killing him then almost everyone living in a developed society is a murderer, since almost all of us, including me, Naomi Novik and most readers of this blog, could save a life by sufficiently reducing our consumption and donating the money saved to some suitable charity. By global standards most of us are, if not billionaires, at least moderately wealthy.
By making the Enclavers guilty of not merely being less aware of their advantages and less generous than they should have been but of becoming rich by crimes literally worse than murder, Novik converts her theme from a defensible criticism of real world inequality to a redo of "The ones who walk away from Omelas." It works to grab the emotions of the reader, as Ursula LeGuin has already demonstrated, but at a cost.