Reading through most comments and I still think a few points are worth mentioning:

* btrfs has a few features that are Really Nice and missing in ZFS: ability of have a file system of mixed drives and adding and removing drives at will, with rebalancing. With ZFS growing a file system is painful and shrinking impossible(? still?). There has been work on it recently though.

* ZFS has a IMO MUCH cleaner design and concepts (pool & filesystems); mirrored by a much cleaner and clearer set of commands. Working with btrfs still feels like an unfinished hack. As human error is still a major concern, this is not a trivial issue.

I _have_ lost data to MD, had scary issues with BTRFS, but never had issues with ZFS in 8+ years. (The fact that FreeNAS is FreeBSD based which I'm less inclined to mess with also means that I mostly leave my appliance alone.)