Suitcase or dirty bombs are not a highly effective way to take out a major US city. The real target is to trigger fear, terror, and mass hysteria. False flag or not, the government will control the narrative and impose airport and border closures, martial law and a curfew.
I am not generally worried about suitcase bombs, its hype. Yes, it would be destructive and cause people to die of radiation fallout, however, the real threat will come from our government and local authority.