Well... There was this thing called Soviet Union. They decided to try to speed up the transition to communism by using repression and violence. And ended up being a totalitarian state, a direct opposite of what a communist state is supposed to be like.
Of course you can argue that Soviet Union was not communist, it was just a state that had chosen to call itself communist for propaganda reasons... But still, Soviet Union is an example of a communist country that was unsuccessful as a communist project already by itself. Then came outsiders and helped make it even worse, but bad doesn't become good by some people wanting it to be even worse.
Burma is another example. I'd say they hacked away their own leg before anyone else, such as CIA, had time to interfere in their business.
Finland decreased its poverty between 1917 and 1991 more than Soviet Union did. In the beginning of year 1917 Finland was a part of the Russian Empire, so we were extremely poor here as well. Soviet Union could be on the second place, perhaps. But, since there is at least one country that fared better, the claim you made it evidently false. There can very well be other countries than just Finland that decreased poverty more than USSR did. I do not know for sure, though, as I'm not terribly well aware of how faraway places like Chile or Burma were faring in 1917.
The USSR had to deal with a civil war, rising up during WWI and being sabotaged by the Germans, more civil war, foreign meddling, and all while being the first successful communist revolution. Yet they still managed to raise literacy, raise health outcomes, raise average life expectancy, gender equality, science and technology, end the cycle of famines (after the first one or two they had when they were still building up), had faster growth during that period than any capitalist country (except maybe the US, which was doing imperialism at the time and the biggest hegemon), all while helping sustain other socialist countries, like Cuba, Venezuela, or North Korea.
The USSR didn't "do repression and violence to speed up Communism," they had a successful revolution and established Socialism. By all accounts it was quite successful overall, but we can learn from where they erred and adapt for the future.
The only ones who believe the Soviet Union wasn't Socialist are generally Western Trots or liberals/Anarchists who already don't want the form of society Marxists want, which is a government that publicly owns its large and key industries and gradually folds in the new firms that grow to that level until the entire economy is publicly owned.
No, the Mensheviks had a poor understanding of Historical Materialism and didn't think the Peasantry could truly be allied to the Proletariat. What I am describing is what the Bolsheviks did. To a better extent the PRC also fulfills this.