The danger is specifically to the copyright holders of Linux - the authors who have code in the kernel. If they do not defend their copyright, then it is not strong and can be broken in certain scenarios.
"Linux copyright holders in the GPL Compliance Project for Linux Developers believe that distribution of ZFS binaries is a GPL violation and infringes Linux's copyright."
Linux bundling ZFS code would bring this text against the GPL: "You may not offer or impose any terms on any Covered Software in Source Code form that alters or restricts the applicable version of [the CDDL]."
Ubuntu distributes ZFS as an out of tree module, which taints the kernel at immediately at installation. Hopefully, this is enough to prevent a great legal challenge.