I personally don’t use Arch, but I think the reason so many people find it stable in practice is because they know their system well. When something breaks or needs to be changed, they know which configuration file to edit, which package to {un,re,}install, what to look for in the AUR, etc., and they can usually avoid those things in the first place, because they went through a fairly hands-on install process, not to mention having the best Linux wiki in existence at their disposal.
On top of that, I think a lot of derivatives of Debian, including Ubuntu and all its derivatives, severely undermine their stability by providing custom configurations for or changes to software that are rarely documented and completely transparent to the user… until they break and leave no indication of how to fix them. Which is one reason why I ended up using base Debian.