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.
ZFS requires a lot of ram to enable on line deduplication.
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.
From the 2nd edition of "The Design and Implementation of the FreeBSD Operating System"
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
"
Even mobile cpus will be all 64 bit before long and it seemed to run great with 4gb others reported success on systems with 2gb. Meanwhile ram available in all sorts of devices is ever increasing.
What fs a laptop/workstation uses shouldn't be determined by whats suitable for a watch in 2016.
