From a political economy standpoint, they're complete idiots. They voluntarily own no land ("We do not Sow"), so they have to rely on forms of primitive accumulation to actually feed their people. The land that they do have is basically useless.
They do own land. They just dont farm it. They have thralls for that. Thats their whole point - that they take what they want, be it food, gold or thralls. Greyjoys have area for farming but its not very good, they have some good mines (i believe tin or copper, cant remember).
The land that they do have is basically useless.
Its not like they can move. There is a reason the Ironborn raided the Riverlands. They were beaten back and kept in check by the King of Westeros such that they can't raid the North, Riverlands, Westerlands or Reach without the other's having to respond. Instead of being able to attack a weak target, they are up against a united realm - until the war of the 5 kings.
Aside from Aegon the Conqueror and a few times that failed rebellions have had to be put down, nobody has tried to invade their islands because nobody wants them.
It wouldn't be worth the effort. They'd never have the support of the common people there. Even the Andals who invaded eventually assimilated.
No wonder they complain about being poor and thus get basically tricked into electing you know who if you have read the books. Learn to grow some grain assholes! They even say in the books that the reason they build longships is that they don't have enough natural resources to build bigger and more powerful ships. The one thing they are good at -- sailing/pillaging-- and they don't even have the capital to be optimal.
If you've ever played EU4 you know how hard it is for a minor power to attack another minor power when they have the protection of greater powers. This guy shows and promises them the great power of old.
Theon takes over a castle he can't hold and hurts some of the few people who actually loved him. And he's probably going to die for a crime that he didn't actually commit.
Theon is hardly to blame for his actions. He was trapped between loyalties as a young impressionable youth. He'd lived under threat of immediate beheading his entire life. Had his father launched his invasion before Theon went to him, the northern lords may have forced Rob to behead him.
Pyke the castle also makes no sense -- why build a fortress on top of a natural barrier? or 4 of them for that matter? It's like adding a moat to a moat. Just build one giant wall in the front. Not to mention, what's with the wooden bridges that presumably rot eventually? Put up a stone walkway idiots! Oh wait, I forgot, you can't pillage a bridge.
You realize stone can crumble right? Their fortress was once a single castle. What you see atop the towers are the remains of what hasn't crumbled into the sea. The bridges are just there to cheaply connect the different parts. It also makes it possible to prevent others from invading the castle as easily even if they are inside its outer walls.
Even their God sucks. In order to truly worship him you have to literally drown yourself? And this society, full of dumb brutes, has figured out resuscitation?
They're sailors. Of course they've figured out how to resuscitate people who've drowned. These people are people who do not fear death by drowning. They fight in fucking armor on their own ships whereas other Westerosi wouldn't do so out of fear of drowning. That gives them an edge in their piracy.
Speaking of healthcare, their main pastime is throwing axes at each other, resulting in tons of ironmen with missing fingers, which for a society in which raiding is the primary economic activity, is just brilliant.
Peacocks also show strength by inflicting a handicap on themselves. Besides, if they're good enough they wont be missing fingers, making them look even stronger to their compatriots. Makes them more respected as captains.
Now, you say: okay, they're bad, but what about Ramsay Bolton? Walder Frey? Look man, you gotta respect that hustle. Bolton has moved up in the world -- legitimization and titles, not so bad for a bastard. Westeros is a brutal place and you gotta admit that Bolton has got game. Now, he's probably gonna get smashed eventually, but he at least accomplished things through intentional schemes. Frey too.
Ramsay didn't do shit. Its all Roose. Ramsay is protected by his father and by low cunning. Walder Frey makes the mistake of taking the blame for the Red Wedding, something Tywin doesnt mind, which lead to widespread murder of Freys. It may be the end of the line.
However, what do we have with House Greyjoy? Bumbling unlikeable shitheads who accomplish nothing and just create headaches for the rest of the Great Houses.
The Greyjoys represent the desire of the Ironborn to return to the 7 kingdoms being separate so that they can do what they do best once again - raid. Harrenhall was a product of the Ironborn doing their thing.