Firstly, it would not be well-received by the otherwise supportive Chinese, for whom a nuclear strike is a crucial red line. Secondly, the wind could carry radioactive contamination onto Russian troops or even into Russian territory, leaving a devastated land of little use to the invaders. Moreover, there is no doubt that a nuclear attack by Russia on Ukraine would not break the Ukrainians' will to defend, as they have nothing left to lose. Third, it is rumored that the United States has promised a devastating conventional strike on the source of any Russian nuclear attack.
This is where we get to the heart of the matter. Western countries must allow Ukraine to strike military targets in Russia with the weapons provided to them. This is especially a problem for the Kharkiv region, which is close to the Russian border. Russia fires from its territory with all its artillery, but, for example, Ukrainian-made self-propelled artillery cannot fire back into Russian territory if loaded with Western ammunition. Other sources of ammunition have largely been depleted.
This is an absurd restriction, and the sooner it is lifted, the better. Sweden has already allowed Ukraine to strike military targets in Russia with their weapons, and now France has also given its permission. If the United States permitted Ukraine to attack Russian bases with the long-range missiles it has provided, it would bring the end of the war closer.