Maybe I'm incorrect, but from personal testing neither of these abilities do damage to targets. I'm not in party, and have tried them on multiple people. Fireball disappears after it connects, but does 0 damage as if just a snowball. Fireball works on mobs just fine, but not players. Plaguebomb will only explode if you immediately hit it the moment you spawn it. If you do not, it simply disappears and leaves a piece of wool behind.