Burrow for headcrab is really underpowered. The first problem is that you can't burrow in a lot of maps in spots that would make sense to burrow in, i.e. the ground, while some maps let you burrow in places you wouldn't expect, such as the carpeted floor. And then the ability to burrow doesn't do anything besides letting you hide or sneak around humans in the open, because you're blocked by cades (and human hitboxes). Practically makes headcrabs useless, as you don't do much damage in the first place, so hiding in between rounds to ambush players isn't as effective as a wraith or apparition, both of which do much more damage. Might as well be a fast headcrab. Now, if the burrow could actually go under cades (and player hitboxes), headcrabs could be used, in niche situations, and by extension, the king headcrab, who essentially never is selected to be used.