The debris thrown up into the stratosphere from our weapons detonating in Russia would likely be enough to start a nuclear winter, not to mention the fallout making everything in the northern hemisphere a lot more radioactive.
Any large exchange of thermonuclear weapons is going to break most supply chains and lead to a great simplification of society and the deaths of at least 90% of the population in the process as industrial farming stops being a thing.
The exchange on the Russian side would be minimal, if any. On the US side, it'll only need to target major cities to be effective. Destroying Moscow and Saint Petersburg would be more than enough. They could use neutron bombs, as they are cleaner, if they still have.
Destroying Moscow and St. Petersburg might be worth losing Paris, London, Washington, New York, and Los Angeles; but it certainly wouldn't worth losing Jerusalem, Mecca, Rome, and other centers of the worlds history. The others also has a vote, you know.
Still, I'd rather not have to move my ski season to the sides of Mount Golgotha near Jerusalem - that glowing snow might still be radioactive after all.
Any large exchange of thermonuclear weapons is going to break most supply chains and lead to a great simplification of society and the deaths of at least 90% of the population in the process as industrial farming stops being a thing.