I absolutely hated gangplanks ult, until i realized it targeted anywhere in the world. Since then its been useful casting it on lanes where they least expect it, help with ganks, defend other lanes when they cant be reached, etc. Nobody wants to even risk getting hit by it because it CAN be devastating, so it is an excellent disrupter and forces people to scatter. You can use it to help encourage enemies to walk into ganks to escape the ult. In a straight up fight its unpredictable and frustrating, but the utility makes up for it.