Scandinavian countries are "socialist" (here I'm using that word to mean the kind of socialism that those countries do) because they are rich, not the other way around. Belgium for example, is very socialist too, in that sense. But only because we were much richer before the oil crises, started borrowing to continue our stupid spending spree when the crises hit, then when things went back to ok end of the 90s we were again too stupid to pay back our debts and instead decided to spend more instead of less.
In that sense socialism is good as long as it doesn't put the country in peril due to the natural inability of politicians to grasp macroeconomy or to make long term decisions.
Now, strict communism and planned economy as in the one applied by the bolcheviks during the russian civil war, is a completely different beast. And it failed so miserably that Lenin (maybe the only intellectual of all the soviet leaders to come) backtracked and reintroduced some market economy in 1921. Why did it fail? Because it was an economic theory that didn't take into account the most basic thing about the economy : behavior and incentives. If as a peasant, the state allows you to keep 4 bags of grain for yourself and confiscates (or buys at state prices, which is basically the same) everything else you produced, what is the point of producing more than 4 bags? That's it. No sane person would work for nothing, I don't even think it's fair to call that greed.