As I have been using the graveyard for 3 days now, I can conclude that the Graveyard is not overpowered on its own, but is effective when used with the Hog Rider and Rage Spell. The hog will distract while the graveyard does heaps of damage to the surrounding Arena Towers. The hog rider usually dies when the tower gets destroyed, due to the defending enemy troops. If the combo is placed incorrectly, (It's supposed to be hog, graveyard, rage) and they place the graveyard first, then the graveyard might die due to a minion hoard or any harmful spells. The graveyard on its own does about as good as a solo hog. It does moderate damage to the tower, but may get defeated by the cards also listed above. I usually prefer to distract the enemy on the other side of where I will attack so they get angry when I attack from the opposite side. The downside of the combination of the Hog Rider, Rage, and the Graveyard, is that it costs 11 elixir to pull off. So you have to defend with good defensive cards until then.
I hoped you liked the review!