It’s extremely rare. Big breaking updates are normally shown in the arch news. Usually they just require a command or two to remove a conflicting package before the update. I think there’s been a few in the last year, but on the flip side I never got a clean distro update on anything but Debian and they usually took a lot more effort to clean up.

Where it may be “unstable” is if a specific program updates (upstream) with some major change or other, whereas another distro might hold off a while.