Q:

Will a nandroid backup save me from a bad upgrade? 

I have a Rezound on 4.03, and verizon has pushed an update to it today.  I have delayed the install because there's little information about it.  What little I have found claim it make the phone unstable. 
I made a nandroid backup, with the default options.  That means:
[Boot], [System], [data], and [cache] were selected. 
[recovery], [sd-ext], and [.android_secure] were not selected. 
My question is, if this OTA update does make my phone worse, will simply restoring the backup pretty much guarantee a seemless "undo"?  Or is it possible the update could change something that I can't revert? 

A:

An update might include a new radio-firmware (see: What is radio firmware?;  basically you can think of it as the "hardware-layer" or an API to give the installed (Android) system access to the hardware), which is not included with a Nandroid backup.  It could also include a "recovery-image" (see: recovery-mode), which you did not include with your backup. 
For the latter, you could simply include it with your Nandroid backup.  The radio-image is a different thing: I know of no method to extract/backup it from a given device.  You could, however, check which radio version is currently installed with your device, and see if you can get a "RUU" (Radio Unit Update) specifically for that.