So why isn't anyone talking about Obliterators???

So after loosing time and time again, why has no one ever discussed the absurd lethality of obliterators??? In my last 3 games against them (in a mixed iron warriors/ death guard soup list), The obliterators killed 600+ pts worth of units. Infantry, dreadnoughts, makes no difference, they kill everything.

Toughness 5*, 2+ save, 5+ invul, deapstrike, mark of slenesh allows them to access a stranger that allows them to shoot twice. So absurdly powerful that let alone the army having deathguard, the obliterators alone can take out a 3rd of my army. Do I just not understand what I'm doing? Or are they just Broken???

Edited toughness