Let's compare it to the situations Ukrainians found themselves in.
They needed to flee the country, but there was a chance border guards would steal their money if they found them carrying a lot. The same would happen if they were found carrying a bag of gold.
If they happened to enter the EU, they wouldn't be able to fly anywhere because you cannot carry a lot of cash and valuables like gold, jewelry, including watches, and cash. AFAIK the legal limit is somewhere around 20k EUR. So, in a situation where you have to migrate with a lot of money, bitcoin is the obvious choice.
If I remember correctly, in the European Union you can travel with up to 10k euro without declaring the money; if you have more than 10k euro with you, you can still travel but you are require to declare the money, if you don't and they discover you, the money will probably be confiscated until when your position is clarified. I think in the USA is pretty much the same but instead of 10k euro is $10k. But as I previously said, we're talking about declaring the money, not as a maximum limit you are allowed to bring with you.
The thing is, a WW3 scenario would bring out desperation and physical assets would likely be overlooked. Aside from Ukraine, try to look back at the wealthy jews during WW2. Not many of them survived as well. Money didn't get them salvation. In case of a nuke warefare, both fiat, gold, and crypto would be less important where you can't exchange them for more essential things. Everyone loses. But fiat and gold did get through two world wars and though it's going to be a lot worse, these assets will also survive a third one. Crypto, which isn't really a physical aspect may or may not.