This seems like a serious plot hole to me. The face hugger in the Alien franchise is almost completely soft, short of its crab like legs.
It seems like a serious evolutionary disadvantage if your means of procreation were to stick your most sensitive appendage into the most dangerous entrance of host. It would be like:
- if instead of biting a wilder beast, the lion attacked it with its eye
- if instead of kicking a lion, the giraffe stuck its head in the lion's mouth
- if instead of punching a biker in the face, I stuck my thumb in the spokes of his moving Harley.
To me, every single enouncter would proceed as follows:
- Facehugger attaches to host
- Host panics, bites down
- Acid melts host's face and chest
- Host dies due to trauma
- Acid burns through hull
- Everything, including host+alien sucked into outer space
- Implanted alien dies too