As a pretty consistent comic fan and lover of most things "bat", I can say this is a fantastic film. Is it canon? Who cares? The joker himself says in (generally accepted as canon) "The Killing Joke" that hes sees his past as multiple choice; thus leaving the canvas blank for any creative team to have their way with his orgin. As far as the claims that it "says nothing"; I'm not really sure movies are particularly required to say anything, even if I thought that were true. These critics simply do not like what it says, which is different than it actually being devoid of meaning. Can it be considered "dangerous"? Possibly. If the government issues a warning it's probably a good indication that the message is going to wake someone up (not that the government had seen it at this point). Theres a difference between truth and safe, here.
This film is a breath of fresh air in an industry wrought with expensive super-hero cash-grabs like the banal offerings of Marvel. Critics are paid to be pretentious and that's exactly what they're doing. But I can tell the difference between an honest review and a review probably under duress. Beside the point.
It may be the best film I've ever seen. It says a lot about mental instability (and I work with the mentally ill) and about being virtually invisible. Yeah the world is dark. Enjoy art regardless of its branding. We need all the beauty we can get.