I have read Marx, and, being a fan of USSR myself, I can say that Marx's most basic assumptions (that of added value, fair price estimate etc.) are dead wrong or at the very least "incomplete". Thing is, he is building the whole system based on these faulted assumptions, and while the system itself looks legit, when basic axioms are wrong, you've got a house with no solid fundament, so imbalance is inevitable. "Das Kapital" being written not as an economy book but as a propaganda pamphlet adds even more to that.
That being said, state-owned economy is just a business on a higher level, with its goal being not profit but sustainability. I can say that modern economy is slowly moving there itself, even faster with the recent technological spurt with regard to automation and digitalization. Because information technology allows for governance of larger structures.
USSR was a great social experiment (although as any social experiment, it costed a lot of lifes, unfortunately), and it advanced humanity a lot. I'm quite sure in a couple of centuries we'll have a modernized version of that, perhaps even on supernational level. We're inevitably moving there. So even if Marx was stupid wrong about almost all of his theory, the part which states communism to be natural historical replacement of capitalism seems legit to me. But we're of course, far from there.