One of the biggest problems with Communism (or any dogma-based society) is, what do you do with those people who don’t subscribe to your ideology?
You either have to change their mind (very difficult), deport them, lock them up, or worse. This is why Communism can work on a small scale (literally a commune) but never works beyond a handful of people. It's also part of what makes Communism fundamentally authoritarian.
Western democratic capitalism seems to have something of an innate immunity to this issue. If you don't agree with the framework, you can choose not to be involved without becoming an existential threat to the whole system; although I suppose that's partly because the vast majority do play along.
The Gulag Archipelago is a great warning against authoritarianism in general, and yes, I have read it.* Whether it's Communism, Fascism, National Socialism, Catholic Orthodoxy in 15th Century Spain, Revolutionary Jacobins, The Islamic Republic of Iran, or Pinochet's Chile — too much concentrated power is a dangerous thing.
*Technically, I listened to the three-volume unabridged audiobook, narrated by Solzhenitsyn's son (with a foreword by JBP), which I can highly recommend.
The issue I have with people like JBP (and certain members of this forum) is when they cry "socialism" or "socialist" at people, groups, or policies that they don't like, and try to equate them with the very worst instances of Communism in practice. Yes, when taken too far, socialism (like most things) is terrible, you'll get no argument from me there. But someone like Bernie Sanders (or even Jeremy Corbyn) isn't advocating for Stalin-Leninism. Their ideas might share some roots (and there are plenty of grounds to criticize them honestly) but it's disingenuous to equate them to Pol Pot. The Communist Manifesto actually has a whole section on how Communism differs from other branches of Socialism — so while all Communists are Socialists, not all Socialists are Communists.
Greta Thunberg's recent anti-capitalist rant is a good case in point. She makes some valid points about Capitalism facilitating the exploitation of people and the environment over the last few hundred years, and I agree with her that economic growth shouldn't be our only priority. But she ignores the benefits that Capitalism has bestowed on the human race in that time. She also
explicitly states that she isn't advocating for a return to "Socialism, Liberalism, Communism, Conservatism, Centrism, you name it", but that doesn't stop certain people from claiming she's a dyed-in-the-wool Marxist.