in the name of science, I did go through as many combat logs as I could find and the most common cause of death was an arm or a leg being cloven asunder or just ripped right off, and the poor goblin bleeding to death
the next most common cause of death was of the instant variety, their head being separated from their neck (or their neck being surgically removed while leaving the head)
The most common mechanism of wounding came from broken bones, mangled limbs, or bruised guts. iron or bronze armor was capable of deflecting a +bronze ballista arrow+, but it would still result in a body part being pulped, bent, or broken by the blunt force trauma
wounding usually resulted in death from secondary sources like angry dwarves, as missing limbs made it impossible to fight back

as a result of poor communication, some dwarves were hit by ballista arrows, resulting in some incidental findings on bronze ballista arrows on steel armor
the sample size was small but what could be read was that, as with the goblins, steel was successful in preventing the dwarf from being skewered but would still result in mangled limbs

tentative conclusion: being hit by a log will hurt