Not too different than any other grid ending catastrophe.
That's in our arsenal. Do we really know what's in Russia's or China's?
We have a pretty good idea.
Plus, we really approached nukes from a COMPLETELY different philosophy than Russia or China. In the 70's and then successive decades, we started making nukes that would last many, many decades and still perform their duties flawlessly. This cost a LOT of money to do. The Russians (and China, through working with Russia) made nukes that were flashy, but only really be serviceable for about 10 years.
We go more for accuracy, they go more for destruction (but in doing so, knocking out their inbounds is a bit easier). Though, with some of Russia's, we better get them in the first 3 stages of defense, vs. after it separates into different warheads (and thus, targets).
This different outlook even extends to the delivery mechanism, in Russia and China, decades old, crappy trucks vs. hardened silos. And extends to the people manning those missiles....paid and professional career soldiers vs. conscripts.
The simple fact is though....we could never knock out all of Russia or China's inbounds, neither could they do it from us. In addition, they could never neutralize all of our delivery methods, nor us them. In the end, the concept of MAD or Mutually Assured Destruction, is the only thing keeping us all from nuking each other.
This same concept does NOT apply to NK, who simply doesn't have the needed number of warheads to get past our defense systems, or the ability to survive and counterstrike. It is apples and oranges here.
Even Russia and China though, have gone to a more average 150kt warhead. Some have up to a 300kt warhead, but the days of 5mt and 10mt missiles being more then norm are long, long gone.
Russia's subs are also getting ancient, and simply put, are corroding. They aren't making new subs at a rate enough to replace them, and basically over HALF of them will be past their service life within 10 years. For subs, this is non-negotiable, as they simply corrode over time. You can't just modernize them like you can with other war vehicles.