The ultimate problem with Marxism is that it makes a fundamental assumption of human nature; the we are a tabula rasa. This is critical to dialectical materialism. However, the situation seems to be somewhat more complicated than that.
Like any account of political economy, Marxism is an incomplete theory. All the while, its adherents assume a religious position towards it. The same may be said for Capitalism. It amazes me how many people actually believe that Rand’s Objectivism is a complete account and all our problems would be solved if we just started practicing it.
Both proposed systems seem to be points on a cline of political and economic dogmatism, offering differing conceptions of The Just Society that reflect the authors’ differing stances to individualism and collectivism.
The problem with State Communism is that it does not even accurately reflect Marxism, hence we have Leninism, Trotskyism and Stalinism.