Maybe this will help you as an explanation why Btrfs requires extra sausage. The post is from Dalto, in response to my question about how to determine which kernel to boot under Btrfs.
There is also another way to put the kernel to boot first, which would look like this with the default kernel:
change in /etc/default/grub:
GRUB_TOP_LEVEL="/boot/vmlinuz-linux"