There isn't any conceivable way that the government would ever get a nuke off against a Perfect Susano'o user.
The construct can fly, which means that after causing whatever destruction you want, you can immediately leave the city or country.
Not seeing how you would be killed in your sleep either. You could use your Perfect Susano'o to fly to any of the remote islands all around the world and rest in those places. If any aircraft follows you, you could just tank all of their bullets and missiles while wiping them out with a slash of the sword or Yasaka Magatama.
A Perfect Susano'o user in real life actively trying to destroy humanity would be the end of the world realistically.