My RAID 1 and RAID 5 combination system is running for a week in very smooth and good condition.
GRUB cannot install on hard drives that setting up with RAID 0 or 5. That means if all your hard drives are setting up with RAID 0 or 5 and Ubuntu is installed on it, GRUB cannot be installed on the MBR.
You should at least make the /boot to be in RAID 1 and the rest can be set to RAID 0 or 5. If you combined RAID 1 and 5 on your system, you may waste some spaces due to RAID 1.