I've really been feeling the M1216 buff honestly. 4 shots is plenty to kill a person as long as you're in range. But I don't know why we don't have a fully-auto shotty, just for the sake of having one. Honestly the between the AA-12, USAS-12, and M1216, I think they've got the market cornered on futuristic shotties. The Saiga has many issues in real life and the KSG is pump which isn't very tactically advantageous. Plus in real life the AA-12 can take a 20-round drum mag, and it has essentially zero recoil, especially compared to a USAS or Saiga.
I think the problem with the M1216 isn't that you can't kill somebody, but that "in range" the M1216 with 4 shots is just as lethal as any other shotgun with one shot. There's hardly any plus side to using the M1216.