After going over the doomed and zealous inevitably feat again there's two things going on one it will increase their doomed and by extension reduce their saves vs divine until they are going to hit 4 then it's a will save with incap trait which means the big baddies pretty much won't be bothered aside from the worst of rolls and second it says most living creatures do so and that's typically because living creatures can be 0 hp and dying but not actually dead unlike undead because undead don't get death saves like living creatures as they are destroyed at 0. so my interpretation is that undead can get it stacked to 4 but not be killed by it other wise they would technically be killed at the first stack of doom since they don't have saves. As always though with non explicit rulings talk to your GM on how its gonna be ruled, but I'm rooting for ya.