I seem to remember an analysis that in the event of an all out nuclear war involving the U.S. and Soviet Union that there would be literally tens of thousands of cities and towns with populations of under 100,000 basically untouched around the world.
Sure, but you've wrecked the economies in the Allied and Warsaw Pact countries (and the world economy as a result), as most GDP is created in and around major urban centers. You've also wrecked their national and regional governments, industrial centers, major transportation and communication nodes, medical centers, and knowledge centers, all of which are located in or near major urban centers.
You've probably also wiped out the bulk of their energy supply, as petroleum production and storage was the other major Cold War target for strategic nukes besides urban centers and governments.
And radiological elements will spread far from blast radii, inducing life-shortening illnesses and cancers and reproductive problems for decades or longer.
You're certainly not wiping out the population or turning the land into a moonscape. But until recovery takes hold, you're basically setting the standard of living for the surviving population back to the 1700-1800s, highly localizing their organization, and adding radiation damage to their health to boot.
And no, don't bring up "nuclear winter" as it is a myth.
It's not a myth. It's an outcome of certain atmospheric models. And like any model, the outcomes depend on the assumptions going in.
If your assumptions are that nuked cities would create firestorms like Hiroshima, that the soot from those firestorms would rise high into the atmosphere, and that rains would not quickly wash the soot out of the atmosphere, then your assumptions will create a long nuclear winter outcome in the model. Studies in the past ten years using modern climate models and these kinds of assumptions verify the hypothesis of nuclear winter in their models.
If your assumptions are that modern cities will not create firestorms like Hiroshima, that soot will not rise too high in the atmosphere, and/or that rains would quickly wash the soot out of the atmosphere, then your assumptions will create a short or no nuclear winter outcome in the model. Critics have pointed out these issues in the past decade but AFAIK have not run models showing a short or no nuclear winter using their preferred assumptions.
Regardless, there's no way to completely verify these assumptions and models without running the experiment. And that is an experiment you do not want to run.
Is this remotely realistic? Apparently nuclear weapons were fairly widespread as they were used in massive numbers in the first and second succession wars, we know the Skye defenders were extremely desperate during the invasion. But how reasonable is it that in fully a century and a half of war that on other planets there were not times that other forces either attacking or defending were desperate enough to use nuclear weapons. Especially in areas where collateral damage was likely to be minimal.
There's a big difference between limited tactical use of nukes in military engagements and an unlimited, widespread, mutual exchange of strategic arsenals. The worry during the Cold War was that the former would lead to the latter.
With few exceptions (the Regulans at the end of Jihad, mainly), the BT universe seems to focus on the former. Even the hundreds or thousands of nukes employed during early Succession Wars were spread across hundreds or thousands of planets. Worlds or even continents weren't cleansed in nuclear fire, as was the Cold War fear. Rather a shipyard here or a water processor there were nuked. This still lead to massive loss of economic/industrial/technological/military capability and the loss of many planets when their terraforming equipment or economic reason for being were destroyed. But the Houses have never feared mutually assured nuclear destruction -- they're just too damn big.
Given this, there may have been some dwindling tactical nuclear use in the early 3rd Succession War that simply didn't get noted in the top-level, in-universe histories that we have access to. I think there's certainly room for some more incidents like the dam on Skye in one's own campaign.
But the trend in political and military thinking in the Houses was clearly towards preservation of their remaining industrial and military base. That ultimately meant no WMDs and more limited military engagements.
