Who created the ubuntu-minimal-16.04-server-armhf-raspberry-pi.img.xz
image?
I found it linked from here, and it's the only Ubuntu 16.04 image I've found that works reliably on both the Raspberry Pi 2 and Raspberry Pi 3, but it doesn't appear to be an official image released by Canonical. Unfortunately, the official Raspberry Pi images published by Canonical here don't work on the Pi 3.