I don't really get this criticism. The story itself isn't particularly incoherent; it's made patently clear at the start. There's the story on the beach, which takes place over the course of the week as the surrounded soldiers wait for their escape. There's the story on the sea, which takes place over the course of a day as the rescue effort reaches its final stage, and there's the story in the air, which takes place during the rescue act. It takes a while to understand if you forget being told it at the start, but once the film reaches its climax it seems pretty clear.
As for characters, there's plenty going on; the captain on the boat who refuses to turn back, and ends up losing a son as a result. The shellshocked soldier who kills him accidentally, and the other son who makes things easier for him by not telling him. On the beach there's the admiral who knows everyone is fecked but remains stoic and resilient all the same. There's the Frenchman who's trying to hide his identity as he does all he can to escape, and Harry Styles' character who catches him out and tries to force him to sacrifice himself.
The characterisation is deliberately bare since it's taking place in the midst of war, but I'd say it's still there and still manages to grip you as the film progresses. Not that the film's necessarily perfect, or doesn't have flaws, but certain criticisms seem fairly lazy.