grub2 can do a lot already, i.e. boot off LVM on RAID6 and other fun
stuff, but there are certain things it cannot do (yet):

  - boot off a partitioned md device (e.g. boot is /dev/md0p1)
  - boot when /boot is somehow encrypted

lilo can't boot off LVM or RAID, but if /boot is RAID1, it can
access the underlying devices directly (read-only).

It would be really nice if d-i included a check for this stuff…