Motion vectors are not scrambled. An attacker seeing only encrypted streams might infer object trajectories. Mitigation: apply jigsaw at picture level instead of slice level (higher overhead).
For a 1080p frame with ~120 slices, the permutation space is ( 120! \approx 2^660 ), far exceeding 128-bit security. The effective entropy is bound by the PRNG state (128 bits). A 128-bit exhaustive search is infeasible. jigsaw x264