> major problems
That's a bit of an understatement. It has a pretty much 0% success rate, for both coercive and voluntary attempts.
> capitalism has always had serious flaws too
Yes, but nothing like the communist failures, and capitalism has had incredible success stories.
> The Soviets had many orders of magnitude more losses in WWII also.
Nobody doubts the heroism and sacrifice of the Red Army in WW2. That's a whole 'nother topic, though.
> The U.S. only barely beat the Soviets in the space race etc while being massively richer.
The USSR was able to direct massive resources at single problems, at the sacrifice at most everything else, but the US could do it with little sacrifice.
> did everything they could to undermine any success
Nobody made the communist countries perform mass slaughter.
> The kibbutzen failed not strictly because all the ideas were unworkable.
They couldn't feed themselves. No communist system has ever been able to feed itself. All have required imports from capitalist economies, or starved, or tolerated capitalist farmers.