Once other thing to think about is that when we use the rod end with one side open, the bolt will wobble about, the hole in the arm will open with continued action, and we may well keep enough load and thus torque on the nut to continue loosening and wind the nut past the retaining features (sheared pin or self locking portion). But if instead, we sandwich the ball between paired arms, once the axial preload on the nut is lost, the torque to further unwind the nut is lost.