I’ve had an NPC equipped with heavy power armor, though the NPC wouldn’t wear the helmet because it was too encumbering. The NPC was fine: nothing short of a zombie hulk could injure them, so even if the NPC’s attacks were slow and didn’t hit very often, the NPC still survived the zombie hordes.
And eventually the NPC’s skills are high enough that they’re hitting zombies despite the encumbrance, and then it’s all gravy.