They don't have the range to get to Europe so we're in the clear, no radiation here.
If Soeul is nuked the downside is there will be a shortage of TV's, ships, cranes, microchips and advanced electronics in general (Samsung is the biggest consumer electronics company in the world).
The fact that Seoul is in a natural bowl and a nuke would wipe it out (remember the 10 million people living there) is also a downside. Oh, and the world wide recession and risk of a bigger war is also of concern.
I don't think it will happen though.