All /targetenemy or /targetnext guarantees is 2 targets will be hit. Even with 30 shaman casting hex, you're only guaranteed 2 targets. And if there's only 2 targets in your frontal "cone of vision" that /tab targeting entails, then you're guaranteed overlap and instant DRs / immunity.

This is why I don't recommend using it for arenas. Open pvp? hell ya its teh way to go. But not when every cast / second counts.