I was looking at using this for my arch zfs-on-root setups, but I've instead just been hacking on /etc/grub.d/10_linux and /lib/initcpio/hooks/zfs to get the boot menu setup I want with grub. I like the simplicity of it this way with less dependencies (especially otherwise needing to use AUR for the zfsbootmenu build or use the pre-built binary blob).
One concern I had with zfsbootmenu was I couldn't figure out how to load microcode. With kexec, zfsbootmenu can only load one image and late loading microcode may be "dangerous" [1]. I don't know practically if that is a real security issue or not. I tried cat'ing my images together as below, but it still didn't work for me:
mv initramfs-linux.img initramfs-linux.img.orig
cat intel-ucode.img initramfs-linux.img.orig > initramfs-linux.img