All I should HAVE to do to convince any pro-Communist is point out the empirical cases against it: the USSR, Venezuala, and every other Communist nation throughout history has failed in every aspect. Economic free-falls, malevolent dictators, genocides, and truly oppressed citizens are obvious markers of a bad idea.
Steven Crowder has an awesome video about communism and why it's awful. I'd suggest looking it up and watching it.
Some disadvantages of communism include strong government control, little motivation for individuals to better themselves, limited individual freedom and protection of inefficient people or producers. Communism is an economic system in which every member of society receives an equal share of the benefits that come from labor. To achieve this, the government controls all production. All goods are owned collectively, and income payment is based on need. Why want that?