But looking at the idea on paper now, it seems obvious what the problem is: it requires unprecedented power to be put in the hands of a few despots with no oversight. What could possibly go wrong?
Since you believe it is such a good idea on paper, perhaps you can describe to us what the idea actually is? Because either it will be blatantly obvious you don't know what the idea is, or it will be blatantly obvious it does not match any of the places you might try to claim it has been tried.
> it requires unprecedented power to be put in the hands of a few despots
A society with power put in the hands of a few despots by definition is not communism, as a society that concentrates power with one group has class rule, and communism by definition requires the absence of class divisions.
Then it is condemned by that too. If "psuedo-communism" kills millions, real Communism is so impractical and unhuman that it collapses so fast into tyranny and despotism that one needs specialized equipment from physics to measure its duration.
This is not to its credit as a practical system to run a country on.
I would posit that while central allocation may have been discredited as an economic pursuit (outside, perhaps, of being something a Friendly AI might be capable of), the basic concept of earning the spoils of one's own labor is alive and well. Heck, we're moving toward it—fewer large corporations with layers of capital-suckling management, and more tiny networks of startups where each founder retains decent amounts of equity in (i.e. profits off of) their work.
In code terms, Capitalism is an interface; it says how entities interact but doesn't specify the nature of the entities. Communism is a very fragile over-engineered over-implemented "we'll call you" framework that's wrong in numerous ways, but management (literally!) is forcing you to use it.
Its an exercise in semantic line drawing as to whether it really counts as "Communism" at all, but its certainly a very different thing than Marx and Engels' Communism. (Leninism is at least as different from Marx and Engels' Communism as modern Western mixed economies are from the same reference point.)
I think the experiments in Spain actually showed that it would work. Remember, anarchocommunism in Spain was violently crushed by fascists... after the Stalinists betrayed them.
Authoritarianism is authoritarianism wherever you go and its fruits are the same.
Which is to say it doesn't scale. At all. Rendering it pretty useless at the kind of scale its adherents postulate it at.
Anarchism need not be seen merely as a state of a society. Anarchism can also be seen as a means, a method of collaborating with people. One can see anarchism everywhere in daily life and in struggles that are not specifically for the abolition of states. The root of the many anarchist political ideologies is the thought that a human being is best when she freely associates with her human beings and acts in collaboration with the world. As a method of being, it is useful even to people who care not to get involved in the grand affairs of the world.
Like many ideas and vegetables, the root of anarchism is perhaps its most useful bit.
AFAIK, every practical implementation of "communism" falls so short of the description, both ideal and transitionally, in things like The Communist Manifesto as to be blatantly lying when they use the name for themselves.
And then you have countries right now like China which blatantly abuse the label. (At the very least: If your country is engaged in capitalism, you are doing it wrong.)
And the same for Christianity or any other religion, but at least they had some positive outcomes.