As for not criticising things we 'don't understand', there are atheists who grew up as religious people and therefore know very well what they are talking about. I would also ask them (assuming they were not socialists/communists) whether they had a detailed understanding of the writings of Marx, Engels, Lenin, etc. and whether they thought it was wrong to critique communism without having this detailed understanding. I think you can criticise things that you not an expert in so long as you have a basic understanding of what you are talking about (which most people seem to lack with regards to communism.) Of course the more you know, the better your critiques are, so researching the idea you're criticising helps, but its not essential.