Never underestimate the power of a boomerang....those things are spooky.
*snicker*
Personally, the way I would justify it is. . . okay, by and large, the Suicide Squad is not composed of people who can stand a chance against a Superman-level threat. This is intentional, and known. Their job isn't saving the day. Their job is *reconnaissance*: when something weird and beyond human is happening, they get sent in to check it out. So why have guys like Slipknot and Captain Boomerang?
Because they are the canaries in the coal mine, and the canary's job is to die. The team pokes its head in, sees what crazy stuff is happening, and if a few members die? Meh, there's more people in the prisons. Then, once Task Force X/ARGUS/whatever knows what is happening, they come up with an actual response.
What makes Captain Boomerang ( and a few others ) special, and Slipknot not so much? Its not that he's super mega awesome. Its that he's gone on suicide missions, and somehow managed not to die. *That* forms the core of the Suicide Squad: people who should be dead already, but through skill and luck and ruthless determination and sheer stubbornness, manage to still be alive and even complete some missions.
Then, advance to the sequel or the post script, and you have two tiers of Squad: the veterans, who have grown together somewhat, as a terrible family of horrible people, and get sent to do the actual "achieve stuff" missions; and the fresh fish, the new "recruits", who get openly treated like the expendable red shirts they are, until they manage to not-die themselves.