Ethnic tensions were a problem in the USSR as well as Yugoslavia because there were certain ethnic groups such as the Croats and Bosnians in Yugoslavia that felt they did not belong in a unified state where they did not share a common religon, a common language and a nationality. You can't blend a bunch of people that share nothing in common in one country. The tensions are bad because some ethnicities see others as threats to their survival in the nation and use unethical methods such as terrorism and genocide to get their way. Serbians killed Bosnian Muslims for example.