For what it's worth, I did a couple of factory resets on my Nexus 5, doing upgrades, and the Google backup worked. The first time, when I went from 4.4 to 5, it lost a contact and two apps that I'd paid for. When I upgraded to 6, the factory reset worked fine. I got a new 5x and the Google backup installed almost everything. All my apps showed up, all my contacts, all my WiFi connections around town.