The Austro-Hungarian Emire was a constitutional monarchy that only lasted from 1867 to the end of WW I in 1918. The Hungarians hated the union from the very beginning, and the territory now contains parts or the whole of seventeen countries, so obviously the diversity didn't work. Not even the Czechs and Slovaks stayed together! This wasn't a good example, but there aren't many. The short lived Nazi German Empire was pretty diverse too, although they made it get less diverse every day.