Let me first answer your questions before giving some more info on the update process.
There is always some risk. But with the default of only doing minor core release you are pretty safe. (E.g. 3.8.1 had close to 100% update success rate) Also you should think of how while being some risk itself the update also protects you from other risks by e.g. fixing security issues.
You can't really answer this question generally. But WordPress does its best to check as much as possible before doing an update to be e.g. sure that file permissions make an update possible. But of course you could imagine situations like a security system that watches for file changes to cause an alarm on auto-update. But this is something specific to your install then. Default WordPress should be fine.
Generally while WordPress does try to roll back a failed update of WordPress itself, there isn't a rollback mechanism for the whole site if anything is broken after the update. At least nothing automatic. But you're backing up your site in regular intervals anyway, right?
WordPress doesn't do any backups on its own, so as already said in 3, your are doing backups, right?
Continuing from here there is some more info plus links on that topic:
Automatic Background Updates have been introduced in WordPress 3.7. By default only minor updates (3.9 to 3.9.1) but not major updates (3.8 to 3.9) are performed automatically. As these updates usually only fix small bugs or security issues stuff is way less likely to break on those.
Anyway you can configure WordPress Updates any way you like. Always update (also major version) or never update. Just the way you like it. Just have a look at this question or the Codex for more info on how to do that: http://codex.wordpress.org/Configuring_Automatic_Background_Updates
If you decide not to use automatic update you can still do it manually. Again you can find more info at the Codex: https://codex.wordpress.org/Updating_WordPress
And as already said before please do regular backups anyway: https://codex.wordpress.org/WordPress_Backups
Unless you are a WordPress Pro and really know what you do I'd recommend sticking with the auto update defaults and not worry too much about stuff breaking as I believe that a lot of thought has been put in it and the advantages outmatch the drawbacks for a standard WordPress install.