What's the situation on btrfs? It was supposed to make ZFS not that big deal anymore on Linux.
RAID1 is implemented to use the oddness of the pid to decide which disk to read from... RAID5/6 have huge performance problems in certain use-cases. Write hole is apparently still a thing.
Quota seems to be still broken or difficult to handle, the tooling is not exactly nice or intuitive (this may change in the future).
Hardware failure handling is nonexistent - it can't detect a broken drive.
Lot's of other gotchas. Just read the mailing list. I've been fighting against btrfs for a while and I'm happy when I'm not having to deal with it. Works probably fine for desktop usage but don't bother stressing it - at the moment it will break horrible.
That was admittedly a few years back, but it's still a long way from production use in many respects, while ZFS has been in production use for years and is well documented, has good tools, and is very robust. After several years of Btrfs problems, I gave up on it and moved to ZFS, initially on Linux and then on FreeBSD due to it being better integrated.
There are definitely things Btrfs can already do that ZFS cannot, like read-writable snapshots, no parent-child dependency (you can create a snapshot and immediately delete the original), growing pools is easier/faster without having to build new arrays or sequentially replace devices and resilver, etc. And just in the last week there's an experimental encryption patch for per subvolume encryption (eventually per file similar to f2fs and ext4), so the situation is heavy development.
The other thing that matters a ton is kernel version, the development is so heavy there are thousands of insertions+deletions per kernel cycle that even in a year it's ordinary to consider those kernels "old" on the list. Most distros by default use old kernels by Btrfs development standards. The exception is Fedora, and openSUSE who do lots of backports explicitly to support Btrfs.
SUSE Linux Enterprise also uses btrfs by default. There's also a bunch of features like booting into snapshots and snapper (snapshots taken before zypper transactions and on regular intervals with decaying granularity) which are in both OpenSUSE and SLE.