I have seen a few other posts about /boot as btrfs.
IIRC mostly it has been problematic in that before grub can read anything from the btrfs file system it must load the btrfs module, thus issues caused.
The default partitioning to date has always been for /boot to be ext4 (at least on the workstation edition) so loading that module with grub before the kernel is loaded is unnecessary.
I would recommend that you at least consider moving /boot to an ext4 partition to simplify the process. This thread shows how that can be accomplished relatively easily.