Nightstalker can shut down an entire team of supers at once. Stormcaller can chain lightning through people to easily get multi kills around corners. They're all OP in their own way. I'm not saying Sunbreaker won't be good, but I don't think it's going to stand head and shoulders above the other two.
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A Nightstalker tethering that many people would be situational. There's a difference between "can" and "will". And Stormcallers take a little while to kill enemies. It's not a super long time, but compared to the instant kills from Sunbreakers? GG, Warlocks. A Nightstalker could potentially stop a Sunbreaker, but that doesn't mean it's more powerful.
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A Titan killing 3 people with one hammer is very situational too. It won't happen often. Nightstalker has the perk where damage is shared between all tethered targets. If you ask me, that seems like the most OP thing in the new subclasses. Shoot one arrow, it tethers 3 or 4 people, you shotgun one and they all die. They can't even run away and Shadowshot has a pretty large radius. It's basically a free multi kill if you Shadowshot into an unsuspecting crowd.