The last time this was brought up I believe there was a problem with prestige stack spamming by the King Beyond the Wall that unbalanced the way their invasion of the lands beneath the Wall event should work. Personally, I don't see how making them tribal will give them any meaningful change apart from changing the name of their listed govern type. In my opinion their inferior holdings, low supply limit for invading armies and succession mechanics already account for making their gameplay unique and different from the feudal lands down south. Despite what their govern type says, you really shouldn't think of them as feudal lords.