You mention Venezuela, but what about India? France, Sweden, Belgium, Canada, Finland, and Denmark are all arguably socialist -- certain much more so than the US.

Any economic system fails when practiced by corrupt or incompetent governments. There are many failed capitalist states, too.

Edit: if you want to talk about scaling, China and India are debatably doing well (or at least improving rapidly), and both countries have 1.3B people.

China had an interesting idea with special economic zones: let people opt out of socialism if they want to, but stay within the country and get basic protections.