EDIT: Kicking is OP.
Not quite at the scale of kicking but punching seems a little powerful also; Not many could punch someone in the head jamming the skull through the brain. In fist fights between unskilled humans this is not uncommon.
Yes, this is addressed by the complex body mod. The main problem is that skulls have a small RELSIZE as Toady remarked to give skulls a notion of hollowness, but it affects gameplay by making skulls very small. As a result they are easily crushed into the brain body part, which is
always fatal. Making the skull RELSIZE larger fixes this issue somewhat. I've used numbers between 50 and 80 instead of the default 20.
However, upping the skull relsize does nothing to fix the problem that if punching people's skulls in is OP,
kicking does even more damage, regularly shattering bones with no kicking skill or at dabbling. This is hard to notice with humanoids because their default MAIN attack is punching, and they only kick if they get opportunity strikes, but is
very apparent with animals such as horses and elephants which only use KICK attacks.
You can test this by taking control of a humanoid in arena mode, and have them aim kick once. Going back to the arena with "C" and watch the battle unfold. The humanoid you took control over will use their last attack, so they will keep kicking. The win ratio is about 90% between unskilled fighters, and normally because the other decided to punch/kick the head as well.
KICK damage is hardcoded, but you can have the kick attack use the PUNCH attack tag instead and that would weaken the attack to less-obscene levels.
The handy thing about the rib mod is now this allows ribs to be jabbed into the other organs as mentioned, resulting in other ways of death besides skull shattering.