Reading the reviews by the professional critics, it seems like the largest complaint about this movie was that it wasn't the movie they wanted.
Granted, there was no consensus as to what that movie was. Every reviewer had their own opinions, some wanted it lighter, some wanted it darker, some wanted it to go more into racial tensions, some wanted it to be more steeped in the dc universe, and others wanted it more bloody.
In the end, this movie was about mental illness, and all the the things that add up to make both a person, and a society, break. That's the story that was told.
If you liked the Netflix show Mindhunter, you will probably enjoy this. Both the concept and pacing are very similar to that show, probably due to pulling from the same sources of inspiration.