CSM's are nuclear, so the damage they do is OK against population, same with any other seeker (antimatter, crystal shard frag)
Most beam weapons, though should not be able to penetrate an atmosphere (eg. anti-protons going through air? Yeah right. Nice aurora effect, maybe makes the population happier from free entertainment
)
DUC? unless the DU shells are 100m across, they'll only make shooting stars.
What happens when you attack a moon though? No protective atmosphere, but still, the DUC, is going to kill maybe one person (the poor sap that it hits in the head. Or if you're lucky take down a building with 1000 people in it.
Perhaps a change so that Beam weapons can only attack domed colonies on No-atmosphere worlds. That's not even really worth it. Just make beams not hurt planets.
This would greatly boost the effectiveness of Weapon platforms, since they'd have to be nuked from a distance or napalmed, but they could carry PDCs and Beam weapons.