Dedup is generally a bad combination for VM ZVOLs, since the first works better with large blocks (suffers from too high metadata overhead with small blocks), while the second works better with reasonably smaller blocks (suffers from read-modify-write cycles on misaligned writes with large blocks). Also dedup for ZVOL may not work efficiently if file system placed on top of it is not strictly aligned and has block size equal to ZVOL block size, in which case copy of the file may get different alignment and so not identified as a copy.
Torrent is usually bad because by default it uses random block download order, which means they are written to disk in random order, which, combined with the fact of ZFS inability to pre-allocate space, means sequential read of the file(s) after that will be practically random. The are also some other workloads that may have the same issue, for example, actively modified databases.