After more games, I think that there definitely shouldn't be stacking modifiers, especially for armor saves. It's too easy to get units on 2+ saves, largely ignoring -1/-2 rend. It means units without -2 or more rend are much worse and less able to do anything useful.
You should be able to get a +1 modifier and that's it--no stacking to negate further modifiers. Being able to get cover, mystic shield, all out defense, finest hour, and any other relevant ability is crazy. We're leaning even more into a world where you MUST have mortal wound output to do any damage, and that's not a good place to be.