@op yes arena is a completely different story. But STH is useful in the challenges too. If you want to see how good he is, go into an arena battle with Lando lead and STH. STH will taunt first and give his entire team (including Lando) 30% turn meter. That means the entire other team will go before any of yours do. Then, you will have to deal with STH taunt and every time you hit him, he will give his team more TM. Which means Lando will be launching nuclear strikes against your team with devastating damage.
STHan's alone won't kill you. But his secondary skills will.