The problem with using nukes against a nuclear power is that you get nuked back.
Now in the case of the US vs Russia or China, or even India vs Pakistan, it'd be a peer exchange where both nations end up relatively equally annihilated and likely fucked for the next century.
But if its like Ukraine vs Russia, Ukraine could maybe get a few good hits in while Russia would absolutely turn Ukraine into a permanently irradiated hellhole to make the ficiticious nation of Belka look like an undisturbed clean paradise.
Its why nuclear (at least strategic ones anyways) will likely only ever be used as deterrent against the other guy's nukes or in the face of some impending doom that's worse than a nuclear holocaust (because why else get yourself nuclear holocausted by launching a nuke if the alternative isn't as bad?)