ARM boards are shit because of nonstandard boot firmware (openfirmware/u-boot/EFI/BIOS tier bootstrap firmware). A good ARM system is the beaglebone black, since it can be booted without the video decoder firmware, and it is widely supported due to industrial use. The raspi is also popular but frankly you already know both are flawed. For a ZFS server, find a device that is explicitly compatible with a standard FreeBSD or FreeNAS image.
It may be better to buy a new or used atom board, since they follow the EFI standard, and some have ECC support, which adds another layer of protection to your filesystem.