Ala Igbo's pretty small and propped up by loads of Western oil dollars, though.
And not to mention that there are a fair number of monarchies which failed, or are failing, as well: Arabia just went through 25 years of famine from 1985-2010 and lost half of its 28 million people; now the Caliph's forces control only about half of the country. Faith and loyalty didn't do much for them.
The Korean Empire imploded in the 1970s and much of it was turned into a biowarfare-blasted ruin by 1990 or so.
The Kingdom of Cambodia suffered it's own disaster in the 1970s, as did Siam....in fact, Siam is still a mess today!
Great Britain fell apart in 1946 after about 20 years of political stagnation and on and off economic depression; it took until the early 1960s for the remnant state to recover to a reasonable level, and it took a lot of Canadian and Australian aid, too.
France nearly fell apart in the 1880s after a German noble stole the throne: now, the monarch has no real power at all and some of the German families are STILL forbidden from office!
The Two Sicilies turned into a dictatorship under the Mad King Umberto III in the '30s.....who was assassinated in 1938, btw. By moderate Monarchists, no less. And it took Sicily proper and Sardinia both about 20 years to fully recover.
And Somalia? Hah! Before 1971, Somalia was a third world country on the verge of total implosion!
And I believe the Norse Kingdoms have been covered already......
And, finally, Greece is an example of a country that actually did better as a democratic republic than a monarchy....lots better, in fact! So, I might add, are Tanzania, Angola, the Gold Coast Republic, and Uruguay.
Really, it's not the type of system alone that counts.....it's how it's executed.