I use cone. I was standing right next to something I wanted to target. I pressed tab. It locked onto an enemy behind it and to the left. I hit tab again. It targeted something behind it and slightly to the right. I angled the camera up so that the only thing on my screen was the enemy directly in front of me, and -- at last -- it targeted that enemy. Usually it's not this bad, but sometimes....
Oh, and we can't forget enemies with hitboxes that you can stand inside of, that become untargetable with Tab while you're inside of them. Great system.