Google has made it easy to enroll in the Android N beta program (too easy in my opinion) which means that a lot of people who aren't prepared to "fix" things when it all goes south are left with unusable devices. Over-the-air updates always have the potential to screw up a phone or tablet, and when you factor in the beta part of this particular one, things can turn ugly.
There are two common issues we're hearing — people aren't getting the OTA to roll back from the preview, or people are getting an update that breaks the OS and aren't able to fix things because they either can't flash an image or never unlocked their bootloader.
It's a hard truth, but beta developer and enthusiast software programs aren't put in place for everyone
The first is easy to fix — be patient. Whether there is an issue with Google pushing the return-to-Marshmallow OTA, or things are just slow, you're not going to be left hanging forever. Relax and spend the time thinking about how beta software may not be for you.
The second is a harder nut to crack. If you never OEM unlocked your Nexus, you have no means to flash a factory restore image. If you're unable to get the tools working to use fastboot and flash an image, this software wasn't something you should have installed — but keep trying. Visit the forums and get the help you need. People are looking for tricks to work around the locked bootloader issue, but if this is your situation you just may be out of luck.
As pointed out by TheStratMan in the comments below, Google has already done what they can do by providing a way to sideload the OTA.
It's a hard truth, but beta developer and enthusiast software programs aren't put in place for everyone. When an official OTA wrecks your stuff, you have people you can contact for repairs. With a beta — that's clearly marked as something for developers and enthusiasts only — you might be out of luck.
We hope Google is working on some solution, but we're not holding our breath. In the meantime, if someone in the community works their way through it, we'll be sure to talk about it.