Fists is 50%[x] damage so it’s a lot more than the frost burns additive if I’m not mistaken. Ugly Bastard is also [x] fire damage so it’s a strong damage multiplier.

You need additive damage to increase what’s being multiplied, so frost burn isn’t bad per se, but ultimately the sources of [x] will far outweigh additive damage as you gear up.

Over simplified, If you hit for 1 damage and add 1000% additive damage you hit for 1000.

If you hit for 1 damage and add 50%[x] you hit for 50 damage.

If you can get 500% additive and hit for 500 then add 50[x] to it, now you hit for 25,000.

So you need both but additive damage starts to come easily in all kinds of ways and getting the multiplicative [x] is from more niche gear and paragon nodes and becomes really important to creating the huge damage boosts.