Hi, libertarian socialist here. I don't consider myself a Marxist, and so any Trotskyists, Maoists, etc. in here are going to strongly disagree with this analysis. Keep in mind this is all hotly debated in the radical left to this day.
Karl Marx's analysis and critique of capitalism is almost 100% on point. His solution is where libertarian socialists don't agree. Marx believed that the transition from capitalism to socialism can only be achieved through a strong state - a dictatorship of the workers. This state will handle all the organization and the bureaucracy, and then once everything is all set, there would be a natural transition where the state would hand over its power. There were a ton of anarchists in the First International (the majority of members actually), and they pushed back on this idea. They found the idea that a state structured dictatorially would willingly give up its power completely ridiculous. And in my opinion they were right, Marx's strong-state model turned out exactly how you would expect, with the likes of the Soviet Union, with Cuba, or with North Korea. Bolsheviks just straightup massacred anarchists who democratically took over their workplaces, completely betraying their own supposed philosophy. Marxist states simply transition into state capitalism.
So what's wrong with communism? It puts too much power in the hands of the state. Even granting that bureaucrats in a communist state could have good intentions, they are still dictating their own personal brand of socialism that a lot of people might not even agree with. Libertarian socialists feel that a state is probably not even necessary, that workers are more than capable of organizing all this themselves with direct democracy, from the ground up. With this model, workers would organize themselves into their own trade unions or communes, and every commune would have the power to democratically choose its own organizational structure, make contracts with other communes, etc. This model is universally succeeded throughout history, while Marx's model has always failed. For good examples, check out the EZLN in Mexico, or the CNT in Spain. In its heyday, the CNT anarchists numbered in the millions and factory production increased.