It's more stable than Debian and more simple in design than Arch.
It basically doesn't do anything, except run your hardware and software, and that's all an OS should do.
superkret
Oh boy, here I go distro-hopping again.
Just kidding - you can pry Slackware from my cold, dead hands.
Either use Stable or Unstable. Testing is actually the most unstable of the three branches, due to how Debian works:
Updated packages are first introduced into Experimental, then into Unstable when they actually build and run. So Unstable is equivalent to Arch's main branch.
Then they automatically enter Testing after a few weeks without anyone reporting a critical bug.
What this means: Testing is the only branch where the decision over what enters isn't made by a human.
If someone notices critical bugs in Testing, the packages may be kicked out of Testing again until the bugs are fixed. So Testing is the only branch where packages can simply disappear when you run an update.
It's also the most insecure branch: When a vulnerability is discovered, the packages in Stable are patched to close it. The packages in Unstable are updated to a new version that closes it. In Testing, the vulnerability stays until the new version eventually migrates down the line again after spending a while in Unstable.
I've run Unstable for years. IMO it's a great rolling release distro with horrible branding.
Slackware works differently than other distros. After a default installation, dependency tracking is pointless because you install its entire repository up front.
If you need something that isn't in the repository, you've got Slackbuilds that work just like Arch's AUR. Or you can use third party repos with their own package managers, semi-official tools with depedency checking, flatpaks or whatever else you want. The point is, how you manage your packages is your choice. The default package manager is just a helpful bash script.