Back in the day I stuck two full invasion forces of goblins, there mounts, and trolls into an arena with an invasion force of zombies from the tower nextdoor. I was trying to determine if better equipment and a few beasts of brute strength would be able to overcome the sheer numbers of the undead.
Results were inconclusive the first but I did learn that necromancers are fucking assholes who can raise bits of the dead being moved past there cages for destruction outside the fortress. Unfortunately while that situation was being cleaned up my squad of marksdwarves finished off whoever won between the two armies fighting it out in the basement and enough time had passed that when I noticed the logs had been buried.
The second time I did this test it was much more successful and I uncovered something interesting, before the most recent update which added fear and many other things the two armies would end up in one of three situations. First situation the goblins with better training and equipment destroy the zombies before they can be disemboweled but not before they have most of there legs broken. Second situation though sheer numbers and difficulty in killing the zombies kill the goblins while taking heavy casualties and losing a significant number of limbs. Situation three had them fighting each other to a stand still.
Based on various other details it's almost certainly to do with the type of weapons the goblins have. Whips got them killed more often then not. Hammers and maces worked alright but it depended on the skill over the user. Spears, swords, and axes tended to work really well.