One reason the Super Mutant don't use those weapons and it's a long shot, is that their unarmed attack does more damage then the guns do (both the Assault Rifle and Chinese Assault Rifle in the Super Mutants inventories is just a slither away from being broken).
They do have bullets
Edit: Brick doesn't seem able to use Eugene either, she'll have it equipped but will revert to using her fists in combat (even if given ammo)
Edit 2: I think there might be some scripting errors involving some FO3 weapons and some npc's which stops them from using certain FO3 weapons. Do I have any mod that could cause this?
Clover won't use Lincoln's Repeater, she'll equip it if it's given to her but won't use it.
Raiders and Talon Company don't seem to be affected by this
