But Linux makes it a faff because the license is incompatible so you have to run it as a 3rd party module and the kernel devs regularly break it. With FreeBSD it’s already there and you know it will work.
That's only partially true.
The license is "compatible" enough to be shipped by distributions rather than kernel.
AFAIK, the thing is: no one has seriously decided to test the license compatibility (as in, test it in court) and everybody's essentially scared of Oracle dragging them through endless legal litigation. Oracle owns most/all the IP that came from Sun Microsystems and while ZFS/OpenZFS is CDDL licensed... Who wants to spend millions in legal fees to find out?
Canonical did (still does?) ship OpenZFS with Ubuntu but maybe they're not big enough for Oracle to go after them (who knows? the lawnmower works in misterious ways).
My zfs never broke under Ubuntu and installation is easy by installing 1 package.