I am comfortable with the wrist movement. That depend on if I am tremolo picking 1 string only or 2 strings (Metallica style power chords). For 1 string, my thumb and finger movement are enough, and I hold the pick pretty loose. But in the second case, the wrist moves, and the pick is held tightly. I am surprised to see players move entire arm and do tremolo picking. They are few but..woah! I am no expert in physics, but in general, it makes sense if movement is kept to minimum while picking because, you will spend lesser energy and maybe, play fast shredding solos for longer period without getting tired. Imagine someone playing Afterlife solo moving the whole arm!!