“Stable” distros often had a long and risky process of upgrading to the next major release.

But you only do that once every 3-4 years, and you have months to carefully plan ahead.

Meanwhile, pacman -Syu could just provide you a major breaking multi-package upgrade of, say, your desktop environment on any random Tuesday. And then your only options are to either roll the dice and accept that update right then, and deal with all the breaking changes it may introduce, or not receive any updates at all, including security fixes, until you do.

I use Arch myself, and it's been pretty troublefree the past 2 years or so, but it was pretty rough around the Plasma 6 release, and there have been other rough periods as well, like when there were bluetooth/wifi and amdgpu regressions in the kernel.

You're confusing two different meanings of stable. When people refer to "stable" distros such as Debian, they are referring to the meaning of stable as "unchanging," not the meaning of stable as "doesn't crash."

A rolling release distro by that definition cannot be stable.

Same, and next year when bcachefs has nailed down the kinks it'll be basically bullet proof with snapshotting prior to its daily updates.

Fair, I'm always scared to upgrade to a new Debian release and the process is always painful. But that happens every couple years and I can take a week or two to look into it before doing it.

Every time I did pacman -Syu it was a Russian roulette. Most of the times everything was fine, but often enough there was a problem and something was broken and I had to spend a couple days figuring out what was the problem.

You upgrade?