I have filed two doc bugs that are fairly easy to fix. GRUB will assemble and boot pretty
much anything these days: md RAID 0, 1, 4, 5, 6; and in addition to ext, it
understands FAT, NTFS, ZFS, HFS+, XFS and Btrfs. It'll even boot from Btrfs subvols,
on a volume configured using data profile single, raid[0,1,10].
There are some limitations for how many devices GRUB may be able to find. It seems like 4
devices is a safe lowest common denominator (accounts for some Btrfs 5+ disk boot