One real danger is that this would legitimize the use of nuclear weapons against non-nuclear states. Nukes haven't been used since WW2. And the small nukes used then really weren't of much greater scale than the firebombing runs already done en masse in that horrible war. But this would be different. A precedent would be set that a nuclear power, if faced in a drawn out quagmire against a non-nuclear power, could use a nuke to end the conflict. The US nukes Tehran to cut the heart out of Iranian resistance. Russia nukes Kyiv to break the back of Ukraine. China nukes Taipei as the opening move to invading the island.

The US nuking Japan never really established the precedent that such an act is OK. Everyone has just accepted that WW2 was a unique situation, and that using nukes at the end of the worst conflict in human history, one that saw the Holocaust, isn't really out of scale for that conflict. Plus the nukes then were relatively small. But if you lob a hydrogen bomb ten or a hundred times the weapon used to destroy Hiroshima? At a time when your nation is absolutely not in a epochal fight for its survival? That's a completely different situation, a solution to a quagmire of the type that the major nuclear powers have frequently encountered.