MAD is only stable if there are two contenders. Not 3, not 10, and certainly not 30.
The hazards to humanity in general only go up as the number of nuke countries increase. Both hazards from deliberate use, and hazards from mistaken or accidental use.
From a strictly US-centric (read US-sefish) POV, once a non-ally gains nukes, they become in effect militarily invincible. Note our current criminal regime has chosen to make war at Iran, not at NK.
There are libraries full of learned commentary on nuclear policy and nuclear proliferation. Plenty of food for thought there.
Absolutely none of the above justifies anything our current criminal regime is doing in Iran. Any claim by them that it was about nukes is a lie.