Supporters of Marxism are famous for the sentiment, “this time it will be different” usually accompanied by “real <Socialism, Communism, Marxism> has never been tried.” The latter may well be true, though the former is almost certainly not.
At issue is the inherent instability of any social system with that much power concentrated in the center – the evolution of the system is in the direction of totaliarianism. No matter how good the intentions of those in charge of a Marxist government, it has force at its core, and (as Friedman said in the linked video) “the people who set out with good intentions are themselves corrupted, and I may add, if they are not corrupted, they are replaced by those with bad intentions, who are more efficient at getting control of the use of force.”
This is neatly summarized by Mark Antro (H/T Dan Mitchell) as: “‘Real Socialism’ has never been tried in the same way that ‘Real Capitalism’ has never been tried. The difference is ‘almost Socialism’ resulted in the impoverishment & death of hundreds of millions of people, while ‘almost Capitalism’ has lifted billions from absolute poverty.”
A particularly apt quote from Thomas Sowell (in Basic Economics): “[We should] look at economic policies and economic systems in terms of the incentives they create, rather than simply the goals they pursue. This means that consequences matter more than intentions—and not just the immediate consequences, but also the longer run repercussions of decisions, policies, and institutions” (H/T Brad Polumbo). In a Marxist government, the incentives for the governing class all run in the direction of increasing tyranny.
Here is another Dan Mitchell post hitting similar themes. The problem with Marxist governments is not so much what they are trying to do (reduce inequality, perhaps?), but how they are trying to do it (by central government force). No nation with statist policies has ever become wealthy, though quite a few wealthy nations have enacted statist policies and become impoverished shortly thereafter.