In the Dune universe, when a laser weapons hits a shield, both are destroyed in a nuclear explosion reaction.
So instead of building nuclear weapons, wouldn’t it be easier to tie a timer and a “parachute” to a laser gun and drop it from orbit onto your enemy’s city?
It’s sort of like the nuclear weapons situation in our world. Everyone who has the weapon know they can use it, but they won’t, unless there’s somone crazy enough cough Muadib cough to do it, because it all but assures complete destruction of your house by the rest of the great houses. It’s an unspoken rule.