Clinkz is the king of backdooring because you can't get rid of him. You can chase Furion away, Clinkz can hit all 3 lanes so fast that you really can't do much about it but hide in your base waiting for him. Though it's very rare to find a Clinkz who is played well.
I give Furion's strength his ability to turn 2v2 into 2v3 early-mid game. That's about it. If he doesn't use his global pressure to create and win skirmishes early game he almots always falls off and his team usually loses. His ability to push lanes and teleport across the map to force enemy TP scrolls certainly helps drag a game out and give his team time to farm, but, other heroes can do that, too.