I am increasingly skeptical as to whether the nukes would actually work if tried, given some of the failures of the start of the Russian campaign, and I suspect

I am increasingly skeptical as to whether the nukes would actually work if tried, given some of the failures of the start of the Russian campaign, and I suspect they also have that concern. If you're brandishing a gun to threaten people and it goes "click" rather than "bang", suddenly your situation collapses. Which means there's an intermediate step: carrying out an above ground nuclear test. This obviously violates the Test Ban Treaty, but is a lower step than just blowing away Kyiv.