What problems do refugees cause in hosting countries?

Lots. You have economic strain, as you are taking in thousands of people that will need government assistance to get started again. Then you may have political strain in how to deal with the refugees and what rights they may have in your country. You may also have racial tensions if the refugees are of a race that is not like in your country. You may also have religious tensions if the refugees are of a religion that is not tolerant or if the dominant religion in your country is not tolerant.