Forum:How do the Reapers ensure that they kill everybody?

I was wondering if anybody had any ideas as to how the Reapers ensured that they killed everybody. What I mean by this is that they don't do something similar to the Covenant in Halo where they glass the planet so it is literally leveled. As can be seen on Feros, the cities that Reapers attack are left relatively intact. Could people not hide in the rubble and scavenge food until the Reapers thought they killed everybody on the planet and left?