What makes a brand launch video actually work?

One claim, carried everywhere. A launch video works when it says the same sharp thing as the homepage, the deck, the press release, and the investor update — so the launch happens through the film rather than around it. The films that fail are technically fine but disconnected: built late, in isolation, saying something the rest of the launch doesn't. Get the single claim right first, then let the film be the most memorable expression of it.