The beef is mainly that the book portrays Galt's Gulch as both a good thing and as something that would actually function, when the real world consequences of trying to run a society like that are that your town fills up with wild bears that destroy everything and eat your pets (https://newrepublic.com/article/159662/libertarian-walks-int...).
It is a good thing and it absolutely can function, but not as a society. Ayn Rand explicitly rejected this idea [1]:
[1]: Ayn Rand Answers: The Best of Her Q&A, Politics and Economics, Libertarianism and Anarchism