Sheep uses cloud-init system to set all the OS environment.
cloud-init permits to install some packages among a list given to the tool.
Even if this option still allowed with the configuration file with cloud-init enable mode, it is not adapted in cloud-init disable mode.
However, it could be really important to enable it in both mode regarding the way it permits some workarounds for missing packages for some devices that could be installed by automation during the first boot of the OS through cloud-init.
For instant grub.cfg is modified in the root_fs/boot, and UUID of root partition and EFI partition are replaced in this file with 4 lines using sed -i for EFI system.
It will be better to use grub-update to do that but there are some errors when using it during installation. Furthermore we must be sure that grub update will work after reboot. grub-update is used to configure the boot with legacy system.