is there any difference between playing as a pirate faction and Ironborn? they both can go raiding and get invasion CBs right? But what about their process for choosing leaders, is it the same?
Mostly. However, Ironborn get something called a "King's Moot" instead of an elected monarchy. Im not too sure about the mechanics of it but the flavor text is pretty cool.
Oh, and pirates are constantly having tiny rebellions called "mutinies" going off. Also, every non-pirate HATES them. Their base lands are dirt poor but they get a special building to compensate for that called a pirate's den. Their religion has no events or special modifiers and its hard to convert a county to it.
On the plus side, pirates are a very independent lot so if you have the military might you can quickly conqueror them one by one and establish yourself as king. They also have alot of rich, diverse, and small neighbors to pillage from. Its alot less story driven and family lines dont mean as much. Im not exactly sure about this but I think your heir needs to have a high baseline martial or combat skill to inherit without your vassals revolting.
Its actually more fun than you'd think. You dont have to worry about becoming independent from The Iron Throne or (if you succeed) about The Iron Throne declaring a de-jure war to subjugate you. You also have ALOT more opportunities to pillage and gather gold and slaves. You're limited to only thralls as a vanilla ironborn. If you're interested you can make an Ironborn Pirate character and be quite meta. Im not sure if any of the usual Ironborn events will stick around but it could be interesting to find out.
You are not limited to thralls as a pirate...
Another issue is that I have at the moment 4 kingdoms, the islands, myr, tyrosh and Lys, and all with medium crown strength, however some of my vassals have done "pirate invasions" of other vassals, which I think should not be possible.
One example is the succession laws, which say that the strongest son inherits, however in my game it always seems to go the the eldest no matter that his younger brother has higher martial ratings and better traits.