It is IMO, or else there's little reason to care. Otherwise it's just surgical and cold and a technical exercise, like most Nolan movies I find. Audiences care about great characters. After you've seen the movie, it's a bit empty and there's not much to talk about other than technical aspects, the structure of the movie and the fantastic soundtrack.
Definitely this, the lack of story and character development can be really annoying if you care about those thing, Dunkerque is not really a movie, it's more of an experience but at the end, the characters aren't growing, there is no twist, no plot, no nothing, it's just survival scene after another etc...
You don't care about the characters because the movie is too intense, there is no moment for the characters to breath and talk about them, you don't know anything about them at the end, the movie needs 30 more minutes to me but I like what I saw, the technical achievement is marvelous but you're right, the movie is kind of empty, you can't talk about it for hours, there is nothing to talk about really...
Edit: I don't think the movie is overrated but it's a one time experience for me, it's not a movie you'll watch a few times, I think TDK is his best work but Dunkerque is really close.
Nolan ranked movies I saw...
1) The Dark Knight
2) Dunkerque
3) Batman Begins
4) Memento
5) Inception
6) Interstellar
7) The Dark Knight Rises