In my personal view, the problem is "1 attack roll type (Accuracy) vs 4 types of defense (Deflection, Fortitude, Will, Reflex ) ".
E.g: https://pillarsofeternity2.wiki.fextralife.com/Vile+Thorns
Currently, we can increase the success rate of the spell's condition effect by increasing the Accuracy. The difference in defense type is just the scores not the types. Should we treat "the Fortitude, Will, Reflex" as second layer like Armor Rating?