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When you get into a team fight you can't just spread out from his ultimate, because it locks onto you, if you flash backwards and he still manages to target you, it will follow. If it bounces to a teammate then now they have to get away from everyone else.
Let me give Fiddle sticks and Rumble as an example of two ults that can very easily single handedly win a team fight. Both of them have massive wind ups. Fiddles has an actual cast time that can be interrupted. And Rumble's is a skill shot that needs to be placed accurately.
What can Brand do? You can CC brand, catch him out, but if you do not 100 to 0 his health he can panic ult at the last second to wipe a team fight. He can flash in to point click hit ult to force a team back. This isn't like Annie where you can flash out of it as she's casting, While Brand is casting, as long as he clicks tha target it will follow through your own flash or dash. All of this would be much more bearable if it wasn't up every single team fight. Karthus ult is an example of what would be an OP ult but it has a long cast time to compensate. Brand needs the same treatment in this meta. I'm not saying nerf his damage, i'm saying his ult ether needs a wider window where your team can engage or it needs to be harder to land.
(note: not a game designer - or even on gameplay, but i AM an ex-plat 5, currently-hardstuck-gold-5 brand one trick)
brand's ult gets wrecked by morg/malz/yasuo/braum, and he becomes less than useless if the latter two sit on their shields until they see the ult coming. i often spend so long waiting for the perfect bounce in a teamfight that i occasionally die too fast to use it.
i guess the other option would be to make the ult whiff when he dies, but to counteract that it would have to bounce much faster, do more damage, or maybe apply multiple burn stacks. idk, i do kinda agree that brand's ult feels uncounterable if he gets even a little ahead, but also one 300IQ player can move out of the teamfight to severely reduce his damage output. in lane, though, i think he's super oppressive and gross, and if he makes you go b at all, he can clear waves so fast that you're gonna lose a lot of ground.
not sure making it a skillshot would be any more effective, cos it'd still hit something in a teamfight pretty easily and to not be truly terrible it'd need to prioritise champs