Arch uses systemd so you haven’t use Arch if you haven’t used systemd. That probably doesn’t change anything. You are welcome to not use systemd I could care less.
Arch is unstable because it ships packages that are brand new compared to Debian stable (not sid) that ships packages that have been tested for 2 years. Debian also used to only be free software but that has changed as of recently. (Stability and security are the exception)
Debian sid is the Debian unstable branch which has little to no testing. Software goes from there into the testing branch before finally making it into stable. By the time that happens its unlikely you will ever find a bug as the vast majority of the bugs have been found.
On the other hand, Arch pulls the packages as soon as possible as its user base prefers newer packages over stability. That’s fine but to say it is somehow more stable is incorrect. For instance, here are some recent issue on Arch:
My point here isn’t to say Arch is bad. My point is that you can’t just leave Arch by itself for years on auto update without issue. Updating Arch often requires reading of changes and manual fixes. Some people enjoy that, others do not.