I mean I'm perfectly fine with a guy who just throws boomerangs, if he actually threw boomerangs. He does almost entirely nothing in the movie. And he's not the only one.
Deadshot: main character, good backstory, good acting, actually does something
Harley Quinn: main character, so-so backstory, decent acting*, actually does something
Diablo: minor character, good backstory, good acting, actually does something
Captain Boomerang: couldn't understand anything he said, does nothing
Killer Croc: couldn't understand anything he said, does nothing
Katana: couldn't understand anything she said, does nothing
Slipknot: Is Slipknot
*Margot Robbie has been largely praised for her portrayal, but she's pretty much copying Arleen Sorkin's version note-for-note. She doesn't bring anything new to the character other than looking good in hot pants. Not that that's a bad thing!