I view it like an over-revved car. Some parts can handle the strain fine, others are over stressed. Depending what aspect you look at, you will get different results.
As the pressure rises, things will start to fail. Some will not cause additional issues, but others will cascade. Predicting when a cascade failure will happen is difficult, however.
I think it’s more the fact that the Russians likely wouldn’t be selling their “good” nukes. They would be selling the old, run-down ones. They would be a large chance they wouldn’t detonate properly.
There’s also a lot of debate on how well the rest of Russia’s nuclear arsenal has been maintained. It’s highly specialist work that can’t easily be verified by non-specialists. I wouldn’t be surprised if a lot of Russian nukes were already non-viable due to corruption affecting maintenance.