Erich Fromm, who was an avowed Marxist (though certainly a peculiar one), has a good critique of Marcuse in The Crisis of Psychoanalysis (essentially on psychoanalytic grounds, but the critique might be valid for other parts of his thought)—can’t give a precise page because epubs don’t really allow to:

What does Marcuse mean by “technical discipline?” Sometimes it sounds as if he is referring only to problems of therapy; but at other times the word “technical” is used to refer to clinical, empirical data. To make a separation between philosophy and analytic theory, on one hand, and psychoanalytic clinical data, on the other, is untenable in a science whose concepts and theories cannot be understood without reference to the clinical phenomena from which they were developed. To construct a “philosophy of psychoanalysis” which ignores its empirical basis must necessarily lead to serious errors in the understanding of the theory. Let me say again that I am not implying that one must be a psychoanalyst or even that one must have been psychoanalyzed in order to discuss problems of psychoanalysis. But in order to make sense of psychoanalytic concepts, a person must have some interest in and capacity to deal with clinical data, individual or social. Marcuse and others insist on handling concepts like regression, narcissism, perversions, etc., while remaining in the world of purely abstract speculation; they are “free” to make fantastic constructions precisely because they have no empirical knowledge against which to check their speculations. Unfortunately, many readers get their information about Freud in this distorted way, not to speak of the serious damage which all muddled thinking inflicts on those exposed to it.

This is not the place to enter into a full discussion of Marcuse’s works dealing with psychoanalysis, Eros and Civilization, One Dimensional Man, and An Essay on Liberation. I shall restrict myself to a few remarks. First of all, Marcuse, while widely read, makes elementary mistakes in presenting Freudian concepts. Thus, for instance, he misunderstands Freud’s “reality principle” and the “pleasure principle” (although at one point he mentions the right quotation), assuming that there are several “reality principles” and asserting that Western civilization is governed by one of them, the “performance principle.” [it goes on through a few more pages]

In this sense, Fromm thinks Marcuse is an idealist who just uses theoretical concepts however he sees fit without caring about their original purpose. This makes him able to make grand statements and speculations about society (some of which honestly do hit home) but at the same time sorely lacking in any empirical basis.

On a personal note, I have read a bit of Marcuse and found him at his best when criticizing western institutions; his chapters on analytic philosophy in One Dimensional Man were a riot to read.