Yeah, that's essentially what I meant by suggesting there might be a "soft cap" in that you might technically be able to add more, but encounter stability or other issues with all the other mods and lighting used in other renders and animations.
I imagine it could be difficult for some devs to justify a potentially jarring drop in image quality for the sake of having 4+ actors in an animation at once. Games like Eternum and Ripples are already pretty heavily pushing the limits of HS2/HS1 from what I understand.
Edit: Or the 3 actor limit is just misinformation I read.