Anecdotal, but I've been running Win10 from the first Insider Preview in fast ring all the way up to this latest 1607_14901.100 (I think that is correct numbers) on my main sig rig you see below and on 4 other machines of mine as well as 3 laptops from my Mom, Dad and Step-Dad and a few more mixed machines of friends/family. I've became somewhat an expert on what all does and does not change/revert each update as well as how to fix them. I have almost all my desired settings merged into a few .reg files(there is a program that will merge any number of .reg files into one big one that I use) to fix almost everything back to my previous and wanted settings and a few .ps1 powershell scripts to remove all the Metro apps that re-installs every update. Once you have these it is as pretty easy and seamless update. It takes me all of 5 minutes to run my .reg files and .ps1 powershell scripts and then a few manual Group Policy changes that will work with reg files too but I do them manual to make sure they take. I always make a copy of my user folder before updating and just copy it back after the update and that takes care of all of my program settings being changed as well as all my game saves and Nvidia Profile settings for each game.
I do all my remote computer updates with Teamviewer and only once have I had to do a full format/re-install on any of the dozen or so machines I keep up with on fast ring W10 updates. And that was some weird error where the start menu and the entire task bar would not work at all. My main rig has a ton of HDD's./SSD's/Storage and I've never had any problems with any partitions being deleted or messed with. As for changing it so you don't have to put in a password to login, right click your start button and hit run and type "control userpasswords2" and uncheck the box up top that says users have to use a password and then you have to verify your passwords and it will auto log you in. If anyone else has any questions about how to fix something they are having trouble with, hit me up and I can probably give you a reg file or powershell script to run to fix it for you. I've had great luck with W10 so I'm happy to help any that want to use it but are having issues. I think I have a reg file or script for about every problem now
Edit: I may be slow to reply though. I'm at work till 11:30PM EST.Edited by Strat79 - 8/17/16 at 12:41pm