* Under monarchy, one person is chosen to rule "at random". Under dictatorship, there is a competition where the most ruthless person gets to rule.
* Under monarchy, the people believe the monarch rules by divine right. Under dictatorship, the dictator rules by fear.
* Monarchies are more stable, meaning the ruler can plan with a long time horizon. Dictators are more likely to siphon resources while the siphoning is good, since they fear a coup.
* Lacking popular legitimacy, a dictator is forced to consider the self-interest and loyalty of their underlings. This leads to extractive and regressive policy. See this excellent video explaining the game theory: https://www.youtube.com/watch?v=rStL7niR7gs
* Under monarchy, criticism is kept in check while maintaining rule of law, via lèse-majesté laws which make it illegal to criticize the monarch. Under dictatorship, criticism is kept in check via repression. That same repression makes the dictator less popular, which triggers more criticism, and thus more repression, in a doom loop.
Monarchy is an imperfect system. A lot comes down to the person who is "randomly" chosen to rule. But I do wonder if monarchy should be considered an option in countries where democracy has been consistently dysfunctional and the population is poorly educated -- Haiti perhaps?
Most successful democracies were monarchies at some point in the past. Maybe it's just a phase of development a country needs to go through -- in order to achieve mass literacy and civics education, if nothing else.