With +2 to every creatures attack and a complete field full of creatures (even with many just 1/1 + 2/0=3/1), one "lethal" is sufficient to do alot of damage each turn against any opponent without a diamond shield. Also, no way should your cards effect be permanent AND stack.
A graveyard deck requires your opponents to kill creatures in order to make it effective.
Not if your using ball lightings and/or fire/pheonix sacrificing. Anyway death has many ways to kill enemy creatures, and graveyards do stack and nothing says you can't have both other creatures, graveyards, and malignant cells and fill up your field a bit quicker then 5 turns.
Some decks might be able to kill you in 5+ turns. But I could see if one surviving till then and getting a near field full of creatures plus +2 attack on all of them, being pretty certain to win without the opponent having a mass scrubber/killer.