The backfist is probably one of the fastest strikes of all. That is why it is great for drawing first blood. If an element of swing can be generated from the shoulder or with a forward lunging motion, it can even be quite powerful and do sufficient damage.
The hammerfist can generate much power very quickly from many directions but is usually easier for the opponent to see and harder to controll in its many powerful applications-plus side is that it doesn't need that much distance to generate power. I think that it is especially suitable for weaker people as well as trained fighters because it is quite natural and does not require much strength or training.
But when i think of striking surfaces, i think that one should be able to strike with almost any part of the forarm, even wrist if one does not hurt oneself at all or minimaly. So anywhere from the fleshy part of hammerfist to the ulna, i believe it's called, to the forarm or elbow(probably one of the most devastating strikes).
The spinning backfist is definately a kind of swing coming not only from the shoulder but elbow also as one should keep the arm bent until after the spin to whip the forarm and fist out close before the moment of impact.
For backfist-Shoulder is powerful, elbow is fast and wrist is an art.
j