Nick Xiao

If you don't like autologin in Armbian Ubuntu desktop, or if yo want to have several desktops to choose, here is how I did this.
Tested on legacy image, but I belive that same setup should work on mainline too.
 
1. Install ubuntu desktop image on MMC (I did it using Etcher)
2. Install display manager. Because of simplicity, I choose LXDM.
   sudo apt-get install lxdm
   
3. Set default session and disable nodm
   Edit /etc/lxdm/lxdm.conf
   Find line:
     session=/usr/bin/startlxde
   Replace it with:
     session=/usr/bin/startxfce4
     
   Fix for XFCE session entry from LXDM session menu:
     Open the /usr/share/xsessions/xfce.desktop file with a text editor.
     Find the name value and replace the space with a dash or hyphen.
     
   Disable nodm:
     sudo systemctl disable nodm
 
4. Reboot.
   
Now you should have login screen, and after login you should have XFCE4 desktop running.
 
Optionally, you can install more desktops:
 
MATE
sudo apt-get install mate-desktop-environment-extras indicator-application atril desktop-base engrampa mate-applets mate-media mate-notification-daemon mate-power-manager mate-screensaver mate-system-monitor mate-themes caja-gksu caja-image-converter caja-open-terminal caja-sendto caja-share caja-wallpaper dconf-editor mate-gnome-main-menu-applet mate-netspeed mate-sensors-applet mate-user-share
 
LXDE
apt-get install lxde lxde-common lxsession lxsession-logout lxlauncher
 
If you did all of this, you can choose to login in XFCE4, Mate, LXDE or OpenBox desktop.
Best regards!