Geography
China is a vast country, with diversegeography. This made it difficult for emperors to effectively control all areas of the empire. For example, the mountainous terrain of the southwest made it difficult for the central government to maintain control over that area.
Size of population
China has a large population. This meant that emperors needed to be able to effectively manage a large number of people, and to ensure that they had enough food and other resources.
Cultural diversity
China is a multi-ethnic country. This meant that emperors needed to be able to manage different ethnic groups, and to prevent conflicts between them.
Economic challenges
China's economy was based on agriculture. This meant that emperors were at the mercy of the weather. A poor harvest could lead to widespread famine, and this could in turn lead to social unrest.
Internal conflicts
China was often plagued by internal conflicts. These conflicts could be between different ethnic groups, between different political factions, or between different social classes. These conflicts made it difficult for emperors to maintain control over the empire.
External threats
China was also often faced with external threats, such as invasions from nomadic tribes. These threats made it difficult for emperors to defend the empire.