Then those people are either noobs or not Fire mains because he's quite powerful. He's effectively a guaranteed combo starter because he can't reasonably die turn 1 without coming back from the dead. He dodges jutsu that try to interrupt or stop him. Fire mains can then pop off mass condition spam with a long combo following Tobi that usually results in the death of one of your ninjas immediately.

He can revive, dodge, deals high damage, self-combos off his own jutsu, and causes Ignition on his standard attack. He's beastly at this stage of the game and Fire mains will love him to death and then second death.

He's getting nerfed though. The nerf already happened we're just waiting for the server to catch up on patches.