Positional Ranks seem pretty easy to abuse.

One of the biggest flaws of positional ranks is if you get off-roled it wont affect your main rank therefore making you not care whether you win or not. I just played a game with an off-role top who was playing Irelia, this person got crushed by a Jax then just split the rest of the game without defending an inhib turret or inhibitor. The Irelia didn't much for the rest of the game not caring whether we won or lost.