Yeah, but it's a single event. With d20, you or one of your party members usually have a chance to react by tossing a heal, or distracting the opponent, or something else. With exploding dice, the damage just comes out of nowhere and pastes you.Usually, in practice, that's multiple rolls too. If someone is Toughness 10 and gets hit by 2D8+2 damage, chances are it takes more than 1D8 to reroll before you're going to get smashed by that. After all, to get 3 Wounds you probably had to get 12 over their Toughness.