Maybe this is just me, but I always thought of the critical spots on goblins/hobgoblins to be fractional amounts of the black heart. If you ever notice when the black gardens heart shot a ball of "darkness liquid" into the giants, they come to life. Adversely, when you break the "darkness container" on a goblin, the darkness pours out and they power down. The more powerful vex have their "darkness containers" protected i.e. minotaur/harpy. Then you have the most powerful vex, who have so much "darkness liquid" in them that their "darkness containers" are massive and partially stick out, hence critical spots on giant boss minotaurs but not on regular sized ones.