Known Problems of FreeBSD's ZFS implementation

i386 and other 32-bit architectures are still definitely not recommended for use with ZFS due to address space issues. If you still want to go that way, please read the ZFSTuningGuide.

tmpfs(5) isn't recommended for use with ZFS because it can suddenly run out of space due to interaction with the ZFS ARC. http://lists.freebsd.org/pipermail/freebsd-stable/2011-January/060867.html