Well, it is. Is so stable that many of Arch users install Arch once and don’t have to format the computer again in years.
Of course you can’t say that Arch is as stable as Debian, cause it’s not. But it’s totally unfair compare these distros, cause the use cases are completely different.
Don’t use a ruler to measure how loud a sound is.
What if you try another interface, like
shutdown -P now
orpoweroff -p
?