The worst case scenarios would be something like Ubuntu being unable to provide compiled modules, but dkms would still be fine. Or the very unlikely ZFS on Linux getting sued, but that would involve a lengthy trial that would allow you to move away from Open ZFS.
"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.
I'm also not positive how easily they could fix the distribution problems given OpenZFS has over a decade of work after the split, but as a whole I'm only discussing use, not distribution.
It is a pain in the ass, but it has been done before.
This is not true. Copyrights (like patents) do not have to be actively defended to remain enforceable.
This is partially true of trademarks, however.