No wallpaper
I successfully built and installed everything from source (including nautilus-ubuntu), but I have no wallpaper on the desktop. I believe it's a nautilus related issue. Any idea what could be causing this and how can I fix it?

I have successfully built Unity as well on my laptop and I'm having the exact same issue. I'm currently building Unity on my desktop. The biggest difference in the build on my desktop is that I went through Chen's list and deleted ALL the Unity-for-Arch packages off my system, while installing the vanilla packages from Arch as needed. I'm wondering if having a fresh build will have any effect. My next project (if I have time) is to look at updating to the Yakkety repos on all the files if they have updates. Perhaps between the two I can figure out why we're getting the black backgound in Unity.
I just finished building Unity on my desktop. The wallpaper works now. Seems like a fresh build helped with the background.
As I have been working on updating my Unity-for-Arch Github page, I found that the source of the no-desktop-background seems to be the upstart-user package. I had stopped working on the packages right around that spot, then logged out of Cinnamon. When I logged back in, I found that my desktop background was gone. Poking around a bit found that when I uninstalled upstart-user, my desktop background reappeared. If I installed it, it disappeared again. I have continued in the build process, and as of right now, my desktop background IS working. I'll update here once I get the full build finished.
gsettings set org.gnome.desktop.background show-desktop-icons false
This will restore your background, but it covers your icons. I'm still looking for the source of the issue and trying to get it fixed.
I have fixed the background issue. There were a couple patches in the nautilus-ubuntu build that were causing the issue. The corrected PKGBUILD is located on my Github page here:
https://github.com/horsemanoffaith/Unity-for-Arch