Only one thing, really. Nuclear missiles do have a fail rate. Indeed, it was estimates some of the Russian designs for the period had a fail of 50% (although this includes wild misses due to guidance failure). Remember, noones actually done a real test of one..!!! (On either side - and the polar shots are in an even worse case!) Also, nukes dont always go off, and when they do they cause fratricide. So the results, while terrible, will be somewhat erratic with some places that should have been incinerated being ok.