Could I get an explanation of why Brand's ult doesn't automatically prioritize champs?

Not asking for buffs or anything, it's just a question I've been wondering about for a while. I understand that it's encouraging AoE combos in the middle of teamfights, as well as keeping him relatively in the middle of things so he'll have some risk to play, but in some situations I've been in I can't land a W, or all I can do is Q and run, yet there's minions nearby so if I ult will just hit the champ with Blaze on it, and I wasted an ult in a useless bid to try and survive. It doesn't really feel powerful unless you apply Blaze to 2/5-3/5 of the team, and doing that at all is quite hard to do outside of a teamfight where there are no nearby minions. It feels more situational than a lot of ults in the game, and I'm not sure if that's really a good thing.