Marxism is the theoretical perfect form of government with the following flaws.
1. It defies human nature and instinct and thus fails. I will work for myself but not for you unless compelled. Marxism compels with brute force, guns and violence and death. About 160 million deaths since 1917, the Russian revolution.
2. The rule of the proletariat becomes the rule of the elite swiftly. The proletariat becomes the ruled again. The Kulaks are killed. They are most annoying creatures to the Marxist elites though they feed Russia. Not a problem for the elites as they controlled the food. The Pheasants starved by design most particularly in the Ukraine.
3. It does not work. Russia was a failed nation with a lot of nukes before it collapsed thanks to Ronald Reagan. It was the largest nation on earth with vast resources yet economically a disaster. That is what Marxism does. Efficiency goes down the tubes, corruption expands. It always fails.
Addendum:
China is an odd case. It was a Marxist failure for many years. It no longer is such and has great success. They pretend to be Marxist but in reality are a capitalistic dictatorship. Oddly their embarrassment of capitalism has made this the most dangerous nation on earth. They now have the money to do bad deeds around the world.
If Marxism worked I would be a Marxist. It does not work and only brings death, destruction and tragedy.
"Power tends to corrupt, and absolute power corrupts absolutely" is a famous 1887 maxim by British historian Lord Acton, warning that as power increases, moral sense decreases. It means that total authority removes accountability, leading to abuse and unethical behavior, as demonstrated throughout history.
He got it correct.