There are two different classes of criticisms directed against Sartre: those concerned with his strictly philosophical work, and those concerned with this political positions. His philosophical work, though often insufficiently discursive and excessive metaphor-driven, was often strikingly insightful, especially his analyses of emotion, mental imagery, and the structure of the psyche.
Later in his life, his philosophical work focused increasingly on political questions, though always through a completely a priori Marxist framework. For some reason or other, this work of his was pretty much a complete failure: He wrote a 900 page book called ‘Critique of Dialectical Reasoning’, which is so much drivel. Even the little prologue that he wrote to it, titled ‘Search for a Method’, was an epoch failure in its own right. He squeezed off a vaguely ok point about Marx charting some kind of middle road between Kierkegaard’s subjectivism and Hegel’s objectivism; but he didn’t even develop that point, and I don’t know that there is to develop.
For some reason, Sartre wasn’t able to make the transition from theoretical philosopher to real-world-oriented political philosopher. That reason, I suspect, is that the Marxist framework he locked himself into simply isn’t very product. Also, he didn’t read any economics or even very much philosophy, and as a result his work became sterile and self-repetitive. He liked to read fiction, and a was himself very good at writing it; but he simply didn’t read serious non-fiction, was beyond all belief uninformed about the actual workings of society, apart from the little bit of half-knowledge that his Marxist orthodoxies dribbled into him. Even when his left-wingism could have been an asset, as when he was discussing the Vietnam war, his Marxism always skewed what he had to say.
He did some work that strictly political in nature, but it was just shrill Marxist sloganeering. Had his political work simply been poor, that would have been forgivable, but he was a hard-line supporter of Stalin, Mao, and Pol Pot, and he personally did much to encourage to the French equivalent of the hippie movement—which, at the time, seemed anti-establishmentarian and bold, but, as we are now seeing, was incredibly short-sighted.
Despite his considerable gifts, he wasn’t very intelligent at mapping his own philosophical work onto the Marxist framework that he would later adopt, even though it actually was applicable. Sartre’s early work was large to the effect that an authentic personal identity is born of a repudiation of pre-existing values, and Marxism is largely to the effect that an authentic social order is about such a repudiation. But Sartre’s Marxist work, while having little basis in anything other than his earlier work, didn’t benefit from the insights contained in it.
Frankly, I think he didn’t quite believe in the Marxism he espoused. This is why it was hard for him, when in his political mode, to say anything intelligible, and this in turn. The extreme views that he took, so I suspect, were no less about him toeing the Marxist line than they were about him trying to give a little zip to his otherwise boring and uninspired political work.
There are Marxist authors whose Marxism works for them. Sartre was not one of them, even though he was a great writer. The reason it didn’t work for him is that his values were not Marxist values. Sartre’s values were actually libertarian in nature: he wanted to live and let live; and Sartre’s liberalism—his actual liberalism, i.e. the liberalism embodied in his early works, not the Marxist liberalism of his later years—was rooted in this value of his. Marxism is not about this. Marxism is not liberal at all. It uses liberal tropes, but only to manipulate and agitate. Sartre didn’t know that, and as a result he was on a fool’s for the entirety of the second (Marxist) half of his career.
The best author to read on this particular topic is the late Jonathan Bowden, who really tracked Sartre’s influence while Sartre was still alive and was totally onto him. Axe: Jonathan Bowden: 9781909606074: Amazon.com: Books