Well it works great, until is doesn't. We're perpetually a few bad decisions from a few possibly deranged actors away from obliterating all of those gains and then some.
Right, and in the meantime nuclear-armed countries mostly get to avoid the horrible, endless churn of death and war and teenagers being sent off to the meat grinder to push some border here or some border there.
We have eliminated warfare between nuclear countries, conflicts have been reduced to nuclear/non-nuclear or proxy warfare, and that's a very solid reduction in suffering.
The bomb was the end of conventional warfare between nuclear nations. MAD has created an era of peace unlike anything our species has ever seen before.