Of course OSs are unstable, in two senses: First, some OSs, somewhat dependent on hardware and software in play, just tend to crash. Second, OSs can be unstable in the sense of changing things; users are, in this sense, actually supremely stable and tend to be quite unhappy when the system decides one day to move menus around, rename programs, change shortcuts, shift around config files, require manual intervention for updates to work, etc. It is this second sense in which Arch tends to be less stable, and distros like Debian and RHEL are extremely stable.