Interestingly, the "Davy Crockett" nuclear rocket had a blast radius in the "building" range, not even quite "city block" range. That's probably why it didn't really catch on. Why use a nuclear weapon when a few, much cheaper mortar rounds would destroy the target just as well and not leave the spot uninhabitable for a long time?
Because the "uninhabitable for a long time" part is actually desired. Bomb Russia into the Middle Age and make sure it will never be a threat again basically was what fueled the Cold War. It will be a problem again when Ukraine finally blows up.
You need a lot of mortar rounds to have the same effect as 10 Tons of TNT let alone the radiation damage.
Well dug in troops can survive a motor round a small nuke a lot less so.
So could a large enough railgun, or laser, or even a large enough marshmallow could take out an entire city. Or even bats: https://en.wikipedia.org/wiki/Bat_bomb
WMD doesn't mean take out an entire city though.
Of course a "large enough" railgun could be a WMD, but they aren't large enough, so they are not WMD.
your payload does not need to be nuclear to take out a city, nor does your payload actually have to do that work if you hit the right target within the city of close enough to it (there are more than enough cities with some very hazardous manufacturer close enough to cause harm )