Nargott
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I call it "mechanics," you, for example, call it "gameplay."
It's a pointless argument...
As I understand it, "mechanics" refers to the fire damage and its effects in both cases (we need to check if they combine), and the compatibility of the two perks in terms of gameplay. And gameplay refers to more global considerations, such as:
1) Engineers without Chaos (or non-engineers) will complain that the ballista is too weak, or engineers with Chaos will have an overpowered ballista (depending on the base ballista stats). It's impossible to find a "golden mean" because of the two variables and the huge range.
2) In Chaos, this perk will only clutter the build if there's no machinery and, especially, if there's no ballista itself.
And that's a huge difference.