When talking to people on the left about the evils of communism, you need to use emotional language. A rational discussion has no effect on them.
They have chosen (deep down) to place feelings as more important then thinking. (of course this is pretty similar to using your left hand or your right hand)
Their thoughts go, I see a person hurting, and I want them not to suffer, so someone must come and help them. Communists say they are going to help all the people. Thus, communism is good.
Communes do work though. I guess the biggest thing is being able to leave is what makes a voluntary commune work. Communes however, usually fall apart when their leader dies or they fall on hard times.
In our future where "stuff" will be mostly free, then our world will look a lot like communism. Or maybe it will like like super-duper-capitalism. It will be hard to tell the difference.
I am just trying to connect the dots. They have been trying to distance themselves from the red's of the 20th century but I can guarantee you that any new aspirations towards Communism will end the same way.
These people have no idea of game theory and how people make choices under different circumstances. Private property is not just a fancy modern way of organizing society, it is literally a life and death question from a point of evolution. Capitalism is natural selection itself, this is how humans became what they are. If Communism had been implemented 10,000 years ago at the last Ice Age, then Humanity would have gone extinct.
After you realize that all attempts at Communism have failed miserably and have created atrocities, then you can highlight the kind of atrocities they have committed. Like having child labor camps in the Soviet Union, or the Gulags where people froze to death and had to eat dirt and feces to survive. Those kinds of things....
Communes don't work either. I have seen literal examples of this. They say that Communism works on small scale, but I disagree even with this. I have even wrote an article about this:
It has to be consistent. It either works entirely, or it doesnt work at all. There is no magic threshold number that says that from now on it doesnt work, but before it there is a utopia.
Most of the times even the minimum threshold which is 2, like a marriage, doesn't work. Only if it's voluntary and based on clear property rights, like no common householde budget, and things like that that make the marriage more voluntary and less exploitive.