I can introduce the issue on Ubuntu 18.10 "Cosmic Cuttlefish" - Alpha amd64 (20180916) and Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180916).
First of all, there is no mdadm in the Ubuntu desktop images so I need to install it manually.
After I installed mdadm, I can install the system on Intel VROC.
However there is an error about grub-install.
I can introduce the issue on Ubuntu 18.10 "Cosmic Cuttlefish" - Alpha amd64 (20180916) and Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180916).
This issue is after LP: #1779685.
First of all, there is no mdadm in the Ubuntu desktop images so I need to install it manually.
After I installed mdadm, I can install the system on Intel VROC.
However there is an error about grub-install.