APFS is targeted to run on low-powered devices like the Apple Watch. It may be that the alternatives cannot be made suitable for such devices: ZFS is famously memory hungry, HAMMER says it's not designed for < 50GB devices.
From Freebsd Mastery: ZFS Pg 135
"For a rough-and-dirty approximation, you can assume that 1 TB of deduplicated data uses about 5 GB of RAM. You can more closely approximate memory needs for your particular data by looking at your data pool and doing some math. We recommend always doing the math and computing how much RAM your data needs, then using the most pessimistic result. If the math gives you a number above 5 GB, use your math. If not, assume 5 GB per terabyte."
Otherwise I think this is like the myth that ZFS requires expensive ECC ram whereas ECC ram is recommended for any filesystem and zfs needs it no more nor less.
ZFS, Pg 549
" However it is not designed for or well suited to run on resource constrained systems using 32 bit CPUs with less than 8 Gbyte of memory and one small nearly full disk, which is typical of many embedded systems "
What fs a laptop/workstation uses shouldn't be determined by whats suitable for a watch in 2016.