Its all about the perception.

I've been running Arch on testing repos and with the linux-mainline kernel for years now and haven't experienced any major breakage at all.

I believe its down to the reputation of Arch Linux - people perceive it as a "bleeding-edge" and "breaking" OS and thus every even minor problem they experience is attributed to the OS and this perception is reinforced.

The same people might be scourging for hours on ubuntuforums because Ubuntu filled up their /boot partition with obsolete kernels and won't boot anymore. But its still perceived as "stable".