Could you please test if "take down" skill is still bugged or not?
It does consider spear damage.
Undead don't take health reduction damage, so take-down doesn't do much good there.
Also, with normal attack you get chance of 'Pulvarize', 'Disable', 'Shield smash' skills used. Which add more damage to the attack.
Damages are not consistent due to following factors:
-randomization
-region where attack hits (torso, head, leg, arm) // this is one more type of randomization
-enemy stats (gear, resistances, defensive ability)
So it's hard to give exact difference between the attack types.
Following is combat data(legendary difficulty) with:
Hector's spear +32% physical damage
+14% physical damage
+30% pierce damage
Take-down level 16 45% reduction to enemy health
187 piercing damage
Here are 3 hits and 3 Take-Downs on Zombie undead
These are 3 hits on sytyr archer