If the system is stuck, you can safely start over albeit you will have to do it manually.
I'm going to say with a high degree of confidence that the devices that provide a fastboot interface and more so all the Nexus devices are mostly unbrickable unless you flash the bootloader, notice that I say bootloader and not the boot image.
So to start from scratch, reboot the device into fastboot once more. You can do so by pressing power and holding the Volume up and down keys all at the same time.
Locate your image files, I will shortly refer to them as
recovery.img from which I am omitting the device specific tag (
grouper in the case of the Nexus 7 for a more generic answer).
Once you locate them just run:
fastboot flash system system.img
fastboot flash boot boot.img
Optionally flash the
phablet-flash -b does for you anyways:
fastboot flash recovery recovery.img
Then boot into recovery
fastboot boot recovery.img
Once in recovery, locate the Ubuntu side of the install, the file should be named
saucy-preinstalled-phablet-armhf.zip (given that you are working on an unflipped model, the flipped/unflipped will make more sense in the following days from the date of this post). Push this so it is deployed by recovery:
adb push saucy-preinstalled-phablet-armhf.zip /sdcard/autodeploy.zip
adb reboot recovery
It should be mostly done after this.