Whenever I create a character to play in the Iron Isles I use the ruler designer to make him a skilled fighter and a brilliant strategist. However, every time I go on a Reaving every single thing that can go wrong does go wrong. I lose every battle, whether it be against a fishing village or a merchant fleet, my courtiers and my character are constantly defeated and wounded/killed, and I'm generally lucky if my band crawls back home with mortal wounds. Is there another trait that governs the chance of success when reaving? I remember it working in previous versions, but I'm not sure what's happening now.
That's just it, reaving's
very risky.
That's the life of the Ironborn.

You roll the dice, you play the game (of iron, not of thrones), you win or you die.
Granted, they
may need to be at least a bit toned down (I had two fighters of the greatest possible expertise, and yet one of them got maimed and the other one died in one same reaving [two of the world's fewest greatest fighters), but I do not think that they should make
major changes.
Reaving's a good choice when you've got too many dynasty members or in general people at court in those little islands - to "cut the numbers" of the population, without necessarily going to war or provoking someone through traditional raiding.
Once I had
only two dynasty members (ruler and her) and it was actually my idiocy to go on raving. When my heir got killed and I got crippled, unable to sire offspring but for sure to rule for at least another decade or so before game over, I actually laughed.
But, having said that,
tiny improvements need to be made, so that
very skilled fighters (strategist shouldn't contribute, at least not for anyone beyond the chief reaver) get hurt and die
at least a little less often than right now.