There are several skills which scale multiplicative with attributes, meaning that 10% from 12 points in Intelligence will boost your 1000 damage fireball to 1100, which goes to 1211 if you also have 2 points in pyrokinetic instead of 1200 if it was additive scaling.
(1000*1.1*1.1 for multiplicative, 1000*1.2 for additive)
The numbers are small here, but the difference gets bigger and bigger to the benefit of multiplicative skills the higher your numbers get. 30 Intelligence and 10 Pyrokinetic would have the fireball deal 3000 damage with multiplicative scaling(1000*2*1.5), while additive scaling would have it deal 2500(1000*2.5)
Skills with additive scaling are the weapon skills and(indirectly) Polymorph as it directly boosts the attribute.
I think the game would be better if either all skills are either multiplicative or additive so they're at least balanced against eachother. It does require a few tweaks here and there as Polymorph for example would be the go-to point investment for multi-element spellcasters because of the intelligence if only for the early levels when you've not yet reached the attribute cap.
If all scaling is multiplicative, the Polymorph skill would require an extra benefit alongside the attribute boost to make investing points into it meaningful beyond the skill requirements. If it's additive, pure-damage boost skills like warfare and pyrokinetic would need a benefit alongside the damage boost to make it competetive with other skills. The Hydrosophist and Geomancy skills are very well designed in this regard and I'd like to see the other skills get a similar treatment.
Situational damage boosts like the huntsman skill are also very nice(even though they're one of the few things multiplying damage yet again on top of other multipliers), you need to use the environment to your advantage with it. Dealing X% of damage dealt as fire to enemies standing on fire/burning for Pyrokinetic would be interesting in my opinion.
When having multiple multipliers of damage like warfare, huntsman, critical hits, or the suggested Pyrokinetic skill the issue is with having to many synergistic skills if it's all multiplicative, which limits the number of effective character builds people can create as everything that doesn't use these synergies to their advantage and relies on additive damage will fall far behind.
A possible compromise would be to have multipliers go from the attribute boosted damage instead of work off eachother, for example:
A 1000 damage fireball boosted to 3000 by 30 Intelligence and 10 Pyrokinetic(1000*2*1.5) and a 50% damage bonus from high-ground thanks to several points in Huntsman would deal 4500(3000*1.5) damage right now. If each multiplier goes off the attribute-boosted damage it would be 4000(1000*2*1.5+((1000*2*1.5)-(1000*2)).
I don't think critical hits should be affected the same way, as most sources of it are quite an investment, Backstab from daggers excluded. In addition to it being an investment, it would just feel and look pretty weird to see a 150% critical hit only deal 20% bonus damage, for example. Same goes for most sources of multiplicative damage turned to additive, but multipliers add a lot of imbalances to the game.
(small side note; the summoner ability "Supercharger" is additive scaling, not multiplicative, while in this case with both the summon and this ability being abilities that cost AP, it'd make a lot of sense for it to be multiplicative as it only gets worse the higher level you get)
