Boomstick skull gives a 5% chance of an enemy unit exploding (quite powerfully) when killed. You get a slightly reduced score for this.

I see the advantage,,,you kill one unit (say a grunt) it blows up taking the rest of their squad with them...nice!

DO NOT USE THIS SKULL ON ANY LEVEL WITH FLOOD INFECTION FORMS!!

Those little swarms contain many "units" meaning an almost guaranteed explosion. They also move very fast so will likely get up close to your units or your base where their explosion can take out units/buildings.

I lost a whole contingent of ODST (over 20 squads) in a chain explosion that started with one load of infection forms. That cluster blew up taking 3 ODST squads and a nearby Flood stalk with it. From the dead stalk came one of those weird black clouds of flood spores that you can't shoot at and it started infecting my troops. They start to turn on each other and in close quarter combat the ensuing exposions completely toasted my entire ODST outfit...I was most unimpressed, then laughed my ass off because (let's face it) That was *** funny!