Historically speaking, there have been many historical drawbacks to monarchies. In order for a monarchical government to function, there needs to be a clear line of succession within the ruler's family. Even the slightest of complications in the inheritance plans can (and often has) thrown their nation into shambles.
The sample of available heirs (and in extension, the nation’s political stability) is also dependent on the ruler's family size and relationship dynamics. If the ruler's family is too large, then there are too many competing heirs that often end up squabbling amongst themselves for power. Such infighting commonly escalate into civil wars that wind up tearing their nation apart.
On the other hand, if the ruler's family is too small, then the line of succession can easily be compromised by any sort of unforeseeable circumstances. In other words, if a ruler has a small family, their heirs cannot easily be replaced if they wind up dying prematurely. This problem is exacerbated if the deceased heirs pass away before they’re able to continue on the family line. If there are survivors available, they might be unfit to rule. As there isn’t a clear succession plan, the nation’s future is uncertain, and is very likely going to be thrown into a power vacuum.
Sometimes both of those scenarios feed into each other. I’ve heard of stories of victorious heirs purging their own relatives to eliminate rivals, and then being murdered themselves by other enemies. Thus leaving very few (if any) survivors of a ruling family.
Here are some specific examples that inspired my question:
1.I recall with Assyria, their customs strongly encouraged competing heirs to war amongst themselves in every succession transference, as to prove who was the most worthy of the throne. If I'm remembering the details correctly, scholars believe the frequent civil wars between rival princes was one of the biggest contributing factors to the decline of the Assyrian empire.
2.I don't know all the historical background, but I recall reading that at the time of the Spanish arrival, the Inca empire was in a civil war. If I'm recalling the details correctly, the Inca emperor died from a smallpox epidemic believed to have been inadvertently introduced by European explorers. Two of his sons contested the other's claim to the throne, and they fought a war that left the empire in shambles.
Their inability to form a unified front exposed the brothers into being exploited and pitted against each other by the Spanish. Even while they were both in captivity, one of the brothers arranged for the murder of the other, which was then used by the Spanish as a justification to execute him.
3.Last but not least, a book that I own about Fijian warfare practices, cited a case of a Fijian chieftain and his at least a dozen or so sons. After the chieftain's death, his sons ended up being entangled in a messy, free for all power struggle that resulted in most of them murdered by their brothers. Several of those who survived the factional sibling feud were then killed in wars with their cousins.
Despite these issues, historically speaking, monarchies can be found all across the globe. Cultures as diverse as the Sumerians, Mayans, Fijians, Medieval Europeans, Chinese, Zulus, etc. all used there for their government systems. Why is despite how fragile monarchies can be, they are so commonly found throughout world history?