The first two are correct. With non-human head shapes, the hair meshes tend to break. Some could be fixed in the post, but we are talking about hundreds of renders potentially, so it isn't feasible in the long run. Last one? Not really, only dForce strand hair sucks as it eats a ridiculous amount of memory. More traditional mesh hair? Easy enough to use.
Now, the other reasons outside of the aliens, namely Chiel (and the rest of the Elai) and Aegis. Chiel is simple. I ended up liking her without hair. When I created the model, I ended up liking her, and that was it. I tried hair on her, but it wasn't her anymore. And because of her, I changed how Elai was.
Aegis. Aegis was supposed to have more visible implants around the head in TRS1, but unfortunately, there weren't enough of them available back then, then I just forgot to add hair when I started rendering the scenes with her, and with 6 GB of VRAM (during TRS1) you really don't want to rerender unless you have to. The rest of the human-shaped ladies have hair (outside of Elai), even in TRS1, unless they were one of the characters, then they might not have. As for now, why does Aegis not have hair? Consistency, and I like her model.