Secondly the predictions Marxism did make, especially about social relationships in the absence of Capitalism, have turned out to be spectacularly wrong.
Thirdly Marxism has become so buried under so many contradictory off-shots and interpretations that it's no longer possible to know what Marxism actually says. There seem to be some points that Marxists more or less agree on, including the infinite plasticity of human character. But on that point Marxism has been disastrously wrong.
Finally, there's nothing 'reflexive' about trying to put together a comprehensive critique. There's nothing inviolate about Marxism. It made some useful points at the time, and it's possible to learn from them. That doesn't mean it should be treated as the final word on social relations.
The fall of capitalism was indeed beyond-scientific prediction. Marx could have calmly stopped by capital and labour crises - and boy, wouldn't be right?
Picking up contradictory and agreement points of presumed followers is like pedantically reading a Bible by each letter - that is not really interesting. As prediction of social "experimentations" go, social theories can only test a couple of them per century - not a great basis for definite inferences.
We may label the obvious Soviet experiment as disastrously wrong, mut we may also give a decent (non-exclusive) credit to Marxism for almost a century of the social-economic-democratic evolution where almost all members of some societies got full economic-social powers and rights. That should not taken for granted - before we know it, we might find ourselves in the "tested" social systems where only rich Bourgeoisie and their servants are overwhelmingly visible, like it was 100 years ago, or it is shown in Latin-American soap operas.
