One bad thing about a monarchy is that it is only as good as the leader, so a poor leader can be disastrous. It is also widely viewed as immoral, and revolt is a strong possibility. Monarchies also promote people based on birth instead of merit.
A competent monarch can do great things for his country, but a bad monarch can cause disaster. Since monarchs have a great deal of power, they have the ability to harm a nation in a way that can take decades or even longer from which to recover. Even though monarchs are trained from a young age to lead, they might lack the insight needed to govern effectively.
Monarchies are often viewed as immoral. The people have no say in who leads, and they are cast as subservient by being born to the wrong parents. This also creates a practical problem, as revolts are common in history. Elected forms of government tend to be more resistant to revolts since the people choose who leads.
When compared to other types of authoritarian governments, monarchy is sometimes said to be inferior. In other types governments, future leaders compete against others, and those who fail rarely take control. Making leadership a reward instead of a birthright can lead to more effective policy and action.