Poxwalkers would gain new models to their unit for free after each kill. You could end up with huge blobs of zombies that just chew through chaff.
Gw nerfed them so you need to pay EVERY time you go above the 20 model limit.
This is a huge problem because poxwalkers need to be at roughly 30 to sustain any damage. At best they only kill half their own in models. So you have an ever evolving unit that will take massive damage, and cost you huge amounts of points a turn to sustain.
This could have been solved by bumping up their cap to 30, or having the max increase each time you bump above max.
None of this nerfed the real problem where you could kill cultists near them and bump up their blob number to like 60. All this did was nerf their normal, non waacfag playstyle.