Coherence is the main measure I'm proposing. It's true that the Soviet Union and the British Empire did not cohere. They no longer exist, and in that sense, they failed. If the Union had not won the Civil War, the US would have been reduced to a rump of itself as a nation-state. Nations and empires that don't want to fail should cohere.

Empires and nation states are not the same thing (although they have similarities). All empires be definition attempt to unite disparate peoples and nationalities, usually under the dominance of one or two groups that initiate the expansion. (Russians in the case of the Soviets; English in the case of the British.)

Nation-states often encompass far fewer peoples. The etymology of the word nation is related to the Latin word for birth, and implies that a people share a lineage. This is especially evident among the smaller nations of Europe, where you often have something close to ethnic and linguistic unity. The breakdown of empires into many nation states often involves purges like we saw with the Armenian genocide (Ottoman), and, in a sense, the Balkan Wars (Hapsburg/Tito's Yugoslavia).

One of the twists, when you're considering African nations, is that most of them had their borders drawn by empires, and almost all of them (maybe all) encompass more than one and often many peoples as different from each other in culture and language as the Spanish are from the Finns. Congo, for example, is as large as western Europe, includes many ethnicities at odds with each other, and given it has been the site of Africa's world war since shortly after the Rwandan genocide in the 90s, you could say that it, too, has failed as a nation-state.