In theory a monarchy could work better than a democracy since the monarch would be inclined to tend for the long-term well being of a nation, opposed to a president who needs to be concerned about elections and public approval etc.
Yes Nikolai II and Louis XVI were possibly the worst monarchy had to offer, but using their rules as an example of why monarchy doesn't work isn't different from pointing out their popular successors, Lenin and Robespierre, wheren't exactly great rulers. It would be more accurate to say that power in the hand of a single person can turn out badly.
I don't see why a constitutional monarchy where, say, the monarch is both the head of state and government, would inherently turn out worse than a US-like republic, or any other state where people still do have a political voice of sorts.
