To answer the OPs question:
Ubuntu didn't work until I added gnome-session to /etc/xrdp/startwm.sh. I commented out the test and exec lines at the bottom of the file and added gnome-session (not sure if that was the correct way to do it or not... but it worked).
It prompts me for access to create a color correction device. I authenticated and had to reboot for the prompts to go away.
If you're going to try other flavors, here is what I found:
I just did a fresh install of 18.04 UbuntuBudgie, Kubuntu, Xubuntu, and Ubuntu. Only Kubuntu worked with XRDP out of the box on bare metal with hardline networking.
Kubuntu gives you a desktop but nags you about a network connections policy. I removed the network widget from the system tray and it worked without hassle after that. This was the smoothest experience of the ones I tried out.
Xubuntu didn't work out of the box (This fixed it - How to configure XRDP to start cinnamon as default desktop session - See the section that has startxfce4 in there which amounts to adding startxfce4 to /etc/xrdp/startwm.sh).
UbuntuBudgie didn't work until I added budgie-desktop to /etc/xrdp/startwm.sh. Again not sure if that was the "correct" way to do it but it worked.