The Khmer Rouge weren’t communists, despite what they may have sometimes claimed. Being funded by the CIA is a useful hint. They were also stopped by socialist Vietnam, a second useful hint.
The USSR was actually very successful, especially during Stalin’s time. It went from a semi-feudal country to defeating the Nazis and into space. Eventually it lost the Cold War to the capitalist countries, through infiltration and direct aggression.
All of Eastern Europe was similarly quite successful compared to the conditions from which each country started. Merely ending the periodic environmental famines through industrialisation and collectivisation was a remarkable achievement. My grandparents and parents describe astonishing change in very few years and better conditions overall than today.
China started from an even less developed stage and industrialised very quickly. The material condition of most workers improved greatly, despite the limits imposed by capitalist countries. Later on, other concessions were made so capitalists would no longer block trade and development. And yet the working class is still in control of the state, as we’ve seen during COVID when the state took more direct control of the economy to save lives at the expense of profits.
The DPRK was bombed to an incredible degree by the US, which is still officially at war with it. Despite an almost complete blockade and even losing its socialist trading partners, it persists and to a small extent prospers. Cuba is in a similar position, doing remarkably well despite capitalist aggression.
Vietnam also developed remarkably quickly, despite occupation and a devastating war. Like China, it made concessions to capitalists in order to develop, while workers retain state power as a class.