For my part, I think the problem with communism isn't so much the economics, (although my limited understand is that mixed approach ala the Nordics works better then full communism or capitalism), but the idea of changing society through a violent revolution. Mass violence, civil war, instability etc radicalize people and play into the hands of ruthless, power hunger people who see anyone who disagrees with them as an enemy to be destroyed. Then when the revolution succeeds you've got a country run by the people best at murdering their opponents and a population that has been through hell and wants safety and someone else to blame.
That's not to say that violent revolution is never justified, but like any other kind of war it should be avoided unless absolutely necessary.