What wasn't helping me here, was that when you do the mathhammer, the average number of wounding rend 0 attacks is about the same for both Herd and Boingrots... but I hadn't taken into account rend, and that's where Herd get better there too. So, yeah, I'm starting to get it now.
(for reference average number of wounds needed to kill a unit:
Boingrots max size: 60 wounds no rend, or 40 wounds at rend 1 or 36 wounds at rend 2.
Herd max size: 57.6 wounds no rend, 48 wounds at rend 1 or 2 or more)
I was trying to find out reasons to use them, if that makes sense
Plus my herd aren't exactly vanilla.....These are my Squig Dice Herd and Fanatics (sadly the photo is a little on the yellow side).