I go along with G. A. Cohen's reasoning on Marxism to start with, namely, that it can be challenged to come up with reasons why it should be treated as having special insights about the human condition, but not just dismissed as another ideology. But it's those reasons which raise the problems. Cohen tried to defend a Marxist claim that the proletariat would come to hold an objective standpoint because their interests would be similar to mankind as a whole. I suggest that sort of defence is dubious to say the least, although I also suspect Marxism is destined to remain influential for a long time to come. The following points (and no doubt others as well) are worth thinking about:
1. The recent history of 'neoliberalism' (deregulation, privatisation and reliance on markets) together with increased inequality appears to endorse Marxism as a way of looking at the world.
2. Cohen points out that a Marxist standpoint views the capitalist as owner of a business, and as alienated (separated) from the activities he controls. The latter idea simply does not apply to most small entrepreneurs (especially self employed) who do in fact work directly in their establishment and 'act upon the world' as Cohen - following Marx - puts it. The separation theme is more accurate for large corporations, but in their case the 'owner' is often a group or even a set of institutions, with varying stakes in the business. Marx was quite correct to point out that capitalist enterprises aim to grow (become big), but to a lesser degree other types of organisation also tend to expand - and become more bureaucratic.
3. Bearing in mind the political behaviour of the working class at various stages in history and in different countries (especially their response to nationalist or patriotic appeals), it seems unlikely that they would ever carry out the universal mission that Marx and his followers hoped for. Indeed, even in times of economic crisis they often did not act as having nothing to lose. Nowadays, the groups who feel in that position appear more likely to try to move somewhere with better prospects than to revolt at home.
4. Perhaps I can accept Marx's ontological view of humans as producers, although I can be uneasy about the environmental implications of that. Whether capitalism can carry on adjusting to that, and the climate crisis, only time will tell.