Sorry to hear that happened.
I agree, an extra warning would be good, and I'll work on getting that put in, along with a few other updates to the plugin.
Undoing is possible, and I've given it some thought in the past, but I just havent had the time, as it's not an insignificant amount of work. Where do you put the backup files? How long do you leave them there? Then there's an interface required to restore them. Keep in mind that they're vulnerable (thats why we updated them!), so leaving them on the server at all is a risk.
I think a better solution might be to, when clicking the "update" button, point out that there's no undo available, and urge them to take a backup right then, just in case.
Lastly - if you can give me some insight into what exactly broke (a theme? Plugin? Which specifically?), I can hopefully update the plugin to avoid the issue in the future.