It has happened to me 2 times in quick succession now that Attila only becomes heir and not ruler when he comes of age (or afterwards, not sure when it happened). This last time Bleda was king and Attila heir on legendary, last time was on hard I believe.
I am not sure of the effect on the huns, they still had super stacks but seemed kind of passive and Attila himself kept mostly to the fog of war so I never got to attack him, making the war vs huns quite a letdown (I was Geats though so maybe that was it). I am afraid I dont have a savegame.
Major bummer when you dont get to fight the Big A-man..