Pve being too easy or too hard hurts any game. Would anyone truely care about the dark souls series if you could run through it as kratos from god of war. Its not that im saying the game need to be really hard, im saying that there is a sweet spot that need to designed for and you design based off of what the player can do and adding in elemental damage primaries throws pve balance out of whack or limits weapon devirsity because weather a select few primaries have it or all primaries can have it, your still gonna be undercutting special and heavy's.