Communism, as applied, has always been married to totalitarian nationalism. In the Soviet Union in particular, you took an impovershed, war-weakened, overstretched empire, killed off the ruling elite, then, out of paranoia of being overthrown, proceeded to eliminate all the educated, intelligent, motivated people that a nation requires to function. Then you try and jump from a 19th century agricultural society to a 20th century industrialized society without having anyone who knows how to do it.
Cuba is an odd case, because the average Cuban's standard of living raised considerably after the revolution - under Bautista, almost all of what Cuba had was being taken by North American business owners and Cubans were left to starve. But between the US trade embargo afterwards and being slaved to the Soviet system, any benefits that could have come out of the revolution were quashed, and Castro's regime was extremely oppressive against dissent and the marginalized.
Vietnam went communist because of Charles DeGaulle. To explain: the French in Indochina (Vietnam, Laos, Cambodia, etc) were aligned with Vichy France, so the Japanese invaders were technically their allies. The Viet Minh anti-Japanese, anti-French partisans were thus on the same side as the Chinese, USA, Soviet Union, etc. At the Potsdam conference, Ho Chi Minh, the leader of the partisans, wanted to meet with the American delegation to talk about Vietnam gaining its independence. Charles DeGaulle, the leader of free France, more or less threatened to pull out of NATO and postwar agreements if we helped the Vietnamese gain their independence from France. Stalin's people, on the other hand, were more than willing to listen, and provide training and Kalashnikovs to the Vietnamese. Now, after the war, the French proceeded to completely fuck up in Indochina. There were thousands of Japanese troops interred, waiting to be repatriated, after the end of the war in the Pacific. The Vietnamese started protesting and rioting for independence, and the French authorities, in their infinite stupidity, decided to arm the Japanese and use them to put down the Vietnamese, who had spent the last 5 years under the brutal subjugation of the Japanese. The US stepped in before it could reignite the war, and shipped the Japanese home. The French then decided to use the French Foreign Legion to keep order, and at the time, their ranks were largely filled with German veterans who enrolled to gain a new identity and avoid war crimes tribunals. Within a decade, the French were soundly defeated at Dien Bien Phu, and they "liberated" Vietnam as a divided state due to rigged elections. The North recognized Ho Chi Minh as the elected leader, while in the South, the French had chosen a leader who would maintain close ties to the former colonial power, much as they did through their African territories. For most Vietnamese people, the issue wasn't one of communism or democracy, but rather one of an independent, self-ruled Vietnam vs one still under the thumb of Euro-American powers. Still, the vietnamese communists did engage in mass murder - more than 4,000 residents of Hue were found in a mass grave after it was liberated. And the Hmong and Montagnard peoples were brutally persecuted by the authorities out of Hanoi.
So, while in a post-scarcity society, communism may be workable, in every historical example so far, it has been combined with oppression of the marginalized, ruthless crackdowns against possible dissent, and an anti-intellectual, dogmatic system that slows or blocks societal advancement.
