What happens if you used "rm -rf *" in /Users or /Users/someUser?
Two cases:


you're logged in as root. Everything goes away.
you're logged in as a normal user.  All of that user's files go away.


In some systems, rm is supposed to prompt you if you do this at the "/" level.  Do not count on that prompt.

With El Capitan, Apple supplies a "prompt" per se, which by default should leave enough of the operating system to boot and recover from your backups.  To see if this has not been disabled, use

csrutil status


Further reading:


How to Disable System Integrity Protection (rootless) in OS X El Capitan
Re: Backing up ACL's