The problem with Thornton is that you can't put him as a Dropback Safety because he's very limited in terms of his change direction ability/agility.
He can really only play close to the LOS as a Torpedo Blitzer.
He doesn't have the range to play as a traditional Safety. Even at Boston College, when I watched his BC film, in 2 years of starting up there, I could count on my hands the number of times he played in the deep 3rds. Tim Lewis pretty purposely hid him from coverage for that very specific reason.
Playing the same role on Miami's Defense as he did at Boston College, basically an extra LB.