I don't know whether it's officially documented, but I vaguely recall being advised, long ago, that a restart of the Mac is highly recommended following uninstallation of ZEVO. Maybe required, if there's to be installation of an alternative.
(The integral uninstaller, typically accessed through the preference pane, might assume that the user simply wishes to uninstall ZEVO … installation of an alternative is a major step beyond.)
http://open-zfs.org/wiki/MacZFS#Notes is edited accordingly; "… a restart of the Mac may be recommended.". If a more cautious form of wording is required there, please go ahead.
Postscript
Part of viewtopic.php?p=485#p485 might fit with my recollection:
dbrady wrote:… Verify that the ZFS IOKit Driver isn't loaded, if it is you'll need to restart. …
… it's not enough to simply unload (or attempt to unload) the KEXT(s). At least in that context, a restart of the Mac was a must.
(Why not simply unload? A bug in the operating system, I guess.)