Name a policy of Sanders that is Socialist and that isn't mainstream in Europe.

I'll wait.

As far as those countries you listed:

Venezuela is still a capitalist country; most of its industry is within the private sector. Its economy and government are in peril because their primary import and economic staple was oil which is now much cheaper than it once was. Russia suffered the same issues from the decline in oil prices.

Venezuela's story is that of mismanagement and sabotage from internal opposition and Western (US) interference.

The United States killed a million Vietnamese in a lost war and is a national shame.

Cuba isn't as wealthy as the US, no. But it has higher HDI than the US, universal healthcare, housing, nutrition, education, employment. Its economic stagnant, but a lot of its misfortune stems from being blockaded by the US. (Yet the US deals arms and trades for oil with a repressive state like Saudi Arabia)

China is an economic powerhouse that combines features of state capitalism/conventional capitalism to build its economy and reinvest in its own economy and industry. It's not really ideal for the working class of China though.