A tactical nuke on Kyiv would be the end of Russia. Aside from the likely NATO response who would be compelled to act, the destruction of Kyiv is almost as bad as destruction of Saint Petersburg or Moscow. The whole mythos and narrative of Russia rising out of Kievan Rus- it’s a key part of Russian identity. Maybe hit anything else but destroy Kyiv, that would Russia destroying itself. Support for the “special operation” would go from highly favorable to near zero. You could almost guarantee a revolution in Russia if they nuked that particular city. It’s like Rome attacking the Vatican.