One says "it is what it is" when no more can be said.
I might possibly understand what you are aiming at. How do definitions have any validity when people have diverse, often incompatible definitions? (But again, we're off the topic here.)
The words in language have meaning by consensus. In the case of objects, the consensus is usually correct. 99% of the population agree on which bridge in the United States is the Golden Gate Bridge and which is the Brooklyn Bridge [100% made up statistic]. True enough, some small town with the name of "Brooklyn" might have a bridge which could be called the Brooklyn Bridge, but no body would confuse some small highway bridge with THE Brooklyn Bridge. [Fact: some small towns are named Brooklyn.]
"Justice" on the other hand is a condition whose meaning maybe only a small number of people will agree with. Everyone wants justice, but who has gotten screwed by the courts and who received justice is (often) in dispute. Should the cop who shot the guy who looked like he had a gun be tried for murder? Opinion will be all over the place. "Justice" will be hard to name. "It is what it is" will definitely not be the case.
There is (or was) a consensus about what the USSR was. It was a union of 15 republics. For all practical purposes it was a single state -- the same what the USA is a single nation, even if it has 50 sub units. It was run by the Communist Party of the USSR, with a very strong executive at the top (a dictator more or less). so on and so forth... The histories of the USSR and the USA are both known. These facts are not open to dispute. "Nations" are collectivities of many people, many points of view, many government units, many industries, and so on. Of course, someone growing up taking care of pigs in the USSR or the USA will have a different take on the country than someone dancing at the Bolshoi or making movies in Hollywood.
But the facts of history are still the same, pig farm or Hollywood.
Someone involved in organized crime in the Soviet Union or the United States will have a radically different POV than somebody engaged in honest work. But the facts of the nations history -- including the fact that some people engaged in crime -- remains the same.
Now, if someone has no idea of what the USSR was (this is possible -- the USSR went out of business almost 30 years ago) then they will just be at a loss to understand what knowledgeable people are saying about the USSR. Someone born and raised in New York City, who has never traveled much, probably doesn't know what a pig farm smells like. Or, for that matter, what a barn for milk cows smells like. That doesn't mean that their understanding of the USSR or the USA is invalid. One could know a lot about the history of both countries and not know what pig shit smells like.
I'm afraid I can't go any further here. My knowledge and interest in the topic is what it is. So, if you want to talk about this more, what you could do is take your opening question to me...
Here's a question: when two people argue about "USSR" or "America," are they arguing about the same USSR or America?
Perhaps the reason they argue so much is because each side is arguing about a different USSR or America (but call their mental representations by the same name).
and start a new thread. You might want to expand those two sentences a bit. Like, adding which Buddhism, or which Christianity people are talking about.