The Biongrot Bounderz Boing! Smash! Ability is stupid good. It's basically the Gaunt Summoner's horde unit killing spell, but on a cavalry unit. Essentially it wipes out 50 percent of any unit of 1 would models.
I get them doing D3 wounds on a charge, but crashing into a unit of 30 Plaguebearers, Witchelves, or Bestigors and killing half of them is ridiculous. It's so good I almost feel they failed to playtest it and it's bound to get nerfed after a year of abuse. Unless the unit is incredibly expensive.