I'm having the same problem. I was unable to get the install to work at all from Windows 10. The installer never saw the Nexus 7, no matter how I configured the USB ports. I eventually gave up and switched to one of my Linux laptops. It seemed to go well, completing all steps of the install and even announcing "Success". But, when the Nexus 7 rebooted, it showed the "Google" logo for a few seconds and then switched to the "Ubuntu" logo. The process bar under the logo immediately jumped to about 50% and then I got the image of a chip with the big "Forbidden" circle/slash behind it, along with the message "This phone needs restoring from a PC or a service center."
Now the Nexus 7 will not boot to Ubuntu Touch, will not boot to Android, and will not even boot to "Recovery Mode." I can't clear the cache or do anything beyond getting the "Dead Chip" screen.
I have tried four or five different USB cables but still have a few more to try. The last attempt I ran was from the command line and I noticed an error message being logged. It said that ADB was unable to find either a device or an emulator. Despite that, the install continued.
Any suggestions would be greatly appreciated at this point.
EDIT: To clarify; only the first install on Linux seemed to go properly. After that, all attempts go through "3 of 12" files, then rapidly count up to 12 before announcing "Success."