Greyjoy’s cards are as powerful as they are one-dimensional. Balon wipes the floor with any other card head-to-head. Victarion routinely plays as a 5 strength card, and it's not too outlandish for him to get up to 7 or 8. Aeron provides a lot of flexibility, helping to not only defend against being baited into playing high cards against weak attacks, but also moving through your hand faster, getting those big hitters back quickly. He’s expensive, and given that Greyjoy has only middling power production potential, that can become an issue. But you still need to keep the premium cards handy, because Asha and Theon suck.
The good thing about these cards is that they are the best in the game at completing the basic task at hand: winning a battle. What they lack is…anything else. They are glamor muscles. They’re probably the worst at inflicting casualties, with Euron’s single sword being the only consistent threat. And they are not any great shakes keeping their own units alive, with Asha’s ability being hindered by Thronemaster’s lack of an option to deny yourself support, and Dagmer still being fairly helpless against both Stark and Lannister’s biggest offensive guns. Furthermore, they have no special abilities that amount to more than a bonus to combat strength. No way to affect the opponent’s house card selection, their order placement, or track positions, as other houses can. They win the battle, and then they sit and wait for the counterattack.
But again, they win most battles. Which they need to, because they take more damage when they lose than they inflict when they win.
KEY CARDS: Balon Greyjoy. With the Valyrian Steel Blade, this sumbitch makes attacking you with less than a +4 combat strength advantage a futile gesture. That's just absurd. Victarion is similarly overpowered, but only on offense. Balon is a powerful deterrent, but mind yourself against Lannister. If Tyrion is still in the mix, it becomes risky to leave Balon as one of your last 2 cards, because he can pull a switcheroo and then you have to go through 6 more battles without the threat of him discouraging your opponents from getting tricky.