It is more difficult for different cultural groups to effectively communicate with each other. Also, diverse cultural groups often disagree about many topics, which often leads to fighting, war, and the inability to get along.
Countries with too much cultural diversity can lose their unique qualities or cause people not to feel pride in their own ethnicity. In Singapore three languages are spoken but 2/3 of the population cannot tell you what the national anthem means because it is not in their language.
When there is a diversity of cultures in a region, you can have ethnic and religious conflict as a result of these different cultures. Diversity is only a strength if it can be utilized to the nation's benefit. The Romans absorbed diverse peoples then used those individuals to help conquer neighboring tribes. A monolithic culture by default has unity.