Closing this bug 'wontfix'. Full-disk software RAID is incompatible with UEFI which almost all amd64 systems include nowadays. Ubuntu 14.04 LTS, while still supported, is not undergoing any further development work, only security fixes; and no further install media will be issued. I do not know if this bug is fixed in later releases of Ubuntu, but ubiquity which embeds grub-installer is also not undergoing further active development, we are in the process of replacing it with a frontend to subiquity. But subiquity will also not want you to use full-disk RAID, again because it's incompatible with UEFI.
Closing this bug 'wontfix'. Full-disk software RAID is incompatible with UEFI which almost all amd64 systems include nowadays. Ubuntu 14.04 LTS, while still supported, is not undergoing any further development work, only security fixes; and no further install media will be issued. I do not know if this bug is fixed in later releases of Ubuntu, but ubiquity which embeds grub-installer is also not undergoing further active development, we are in the process of replacing it with a frontend to subiquity. But subiquity will also not want you to use full-disk RAID, again because it's incompatible with UEFI.