Never had issues factory resetting the XB6. It's a disaster with the XB7 and I've been told things over the phone which don't make sense and lead me to think there's something amiss on Comcast's end.
Normally, after a factory reset and you open your browser, you're taken to a page where you can change the name of your network, your password, etc. Then it sets up WiFi and you're done. With my XB7, I reset the gateway and could not get to the page to rename the network and put in my password. That page flashed in the background and all I got was a white page saying I had to connect, but there was no way to do it.
After who knows how long on the phone with a Comcast CSR, I was told Comcast has to re-authorize the gateway after each reset and the network name has to be changed (can't be what was previously used) and they have to do it. Now, my WiFi network has a new name, but the ethernet connection to my desktop is called "XFSETUP-****.
This doesn't sound right. When I first got the XB7, setting it up was a breeze. Now, I'm concerned that I'll have to go through a bunch of nonsense in the future should I need to factory reset again.