Note that the listed probabilities of surviving N-disk failures are not correct, though after calculation, though the differences may not be that important.

Listed survival probability of N-disk failure in 8-drive/4-vdev mirror.

1: 1; 2: 0.857; 3: 0.667; 4: 0.400; 5: 0; 6: N/A; 7: N/A; 8: N/A

Proper survival probability:

1: 1; 2: 0.857; 3: 0.571; 4: 0.229; 5: 0; 6: 0; 7: 0; 8: 0

Comparatively, survival probability for 8-drive RAIDz4 with equal amount of usable space:

1: 1; 2: 1; 3: 1; 4: 1; 5: 0; 6: 0; 7: 0; 8: 0

Personally, I'd use multi-vdev mirror pools only for data that is either backed-up or data I can afford to lose completely.