FreeBSD and "Linuxy" code
often time FreeBSD (and the other BSDs) might lag behind others because developers often ignore their existence and write "Linuxy" code, code that makes assumptions that you're ONLY going to run it on Linux. That's not very POSIX of them, but it happens a LOT.
And FreeBSD has had a good strong working ZFS implementation running for a LONG time now. So porting FROM FreeBSD's implementation would make good sense.
However, two split ZFS code branches appeared instead. One, FreeBSD's ZFS implementation (which I've been using for several years now), and OpenZFS, apparently targeting Linux. But which one got "most of the love"? The 'Linuxy" one, apparently...
Yet, maybe Linux could benefit from some FreeBSD ZFS features, too. (As far as OpenZFS features go, I'm actually pretty happy with how it is on FreeBSD at the moment so "meh").
Here's one thing FreeBSD has done VERY right with ZFS: A few years ago FreeBSD more or less perfected "boot into ZFS" so you could have a pure ZFS system. But if you want, you can still boot UFS and have ZFS volumes as well. Setting the system up for "all ZFS" or UFS boot is pretty easy. In fact, pure ZFS is a bit easier (for new installs).
I've seen references to "boot into ZFS" being done in Linux, but everything I've read so far seems "hacky" and may be "not officially supported" by the distro (or anything else for that matter).