[witrak]

>ZFS's major failing is that it offers no way to address inevitable filesystem data and free space fragmentation, and while you can remove devices from a ZFS pool, it incurs a permanent performance penalty, because they work around ZFS's architectural inflexibilities by adding a mapping table so it can find the moved chunks of data.

I'm not an FS specialist but by chance a couple of days ago I found an interesting discussion about the reliability of SSD disks where there was a strong warning about the extreme wearing of commercial SSDs by ZFS (up to suggestion to never use ZFS unless you have heavy duty/raid version of SSD disks). So ZFS also has unfinished work and not only the mentioned by you software improvements-related.

BTW From my - nonspecialist - point of view, it is easier to resist the urge to use unreliable features of Btrfs than to replace a bunch of SSD drives. At least if you pay for them from your own pocket.