Battle Brothers has a similar armor system.
Armor should deflect blows or mitigate damage, armor as a secondary hp bar which makes you immune to lots of powers is silly. You can't make a "DC caster" thanks to this shit. Imagine that you are playing EverQuest, Icewind dale or any game as a Gnome illusionist/Enchanter,
can you imagine having to spam fireballs before you attempt to charm/dominate the "goy"? Or playing as a Ventrue in VtMB with such mechanic? Ohh before I use presence or domination in the enemy I need to empty my Desert Eagle in his head, after this I can dominate him.
That mechanic is nonsensical and kills save or else spells.
5E already nerfed such spells a lot and gave legendary resistance which is a way to take out a bit of the RNG of the game. In Kingmaker, there is a guy who destroyed Spawn of Rovagug with a mere tier 4 "phantasmal killer" spell :
https://steamcommunity.com/app/640820/discussions/0/1645418344567598037/
With 41 illusion DC(achievable, see the steam link above), on normal, he needs to hit 13 in a d20 to pass the fortitude save and 15 the will save meaning about 42% chances of instantly dying to a gnome illusionist. If you have a party tanking while you spam spells, using all tier 9 spell slots, that is about 97% chances of killing him before running out of spells.