It suffers from the same problems that any movie has when the cast gets too bloated. There were too many subplots going on at once to properly follow any of them, and they resolved too quickly to feel invested in them.
Combined with essentially zombies because zombies, Joker never getting enough screen time to be anything other than cringe because he looks incredibly out of place, HORRIBLE dialogue, wayyyyyyy toooo muuucchhhh sloooowwww moooooo, and terrible plot mcguffins, it was average at best, boring more often than not.
Margot Robbie was good doe, Will Smith was Will Smith in Deadshot cosplay, and Viola Davis can still play a good morally questionable character, but that's about it in terms of enjoyable stuff on screen.
Also, Will Smith carries the Deadshot mask with him the entire movie, but never puts it on except once in a flashback.