In my experience the ranged brothers are the most powerful ones, especially when they are experienced and the group is fighting against lightly armored enemies. They can kill many before they even reach the front lines. After that they can switch to billhooks or other distance weapons to help out in melee combat. They are less efficient against armored enemies, but with crossbows they can help out a lot, so they stay relevant even in late game.
In short – they range from being the most important members of the group to being useful enough to not consider not having them. Melee brothers range from being near useless meatshields to being valuable members, but not to that extent and they are always taking higher risk when in combat (so you’re much more likely to lose them).
