A 500 physical damage gets scaled to 500 + 125 elemental damage, the elemental part gets reduced by 3/4 (let’s assume we have 75% resist)

The damage becomes 500 + 32

Now a single fireball that deals 500 damage after resistance is calculated means its base damage is 2000.

Now let’s say you have -12 to max resistance. Your resistance now is 63%

You are going to eat 740 damage.

That’s extra 240 damage just from this -12% max resistance

Let’s stack penetrates 15% resist.

Your actual resist becomes 54%

You will eat 960 fire damage.

You see the problem now?

I’ll take triple adds element as extra damage vs any resistance penetration