When comparing Slackware vs Adélie Linux, the Slant community recommends Slackware for most people. In the question“What are the best Linux distros that don't use systemd?” Slackware is ranked 2nd while Adélie Linux is ranked 41st. The most important reason people chose Slackware is:
Uses stable, plain-vanilla packages from upstream.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Stable
Uses stable, plain-vanilla packages from upstream.
Pro Strong adherence to UNIX Principals
Pro Ideal to learn more of Linux
Follows the original Linux roots. It still sees Linux as a free clone of UNIX so the distribution tries to be UNIX-like.
Pro Package management
Uses standard tar archives with shell scripting as packages.
Pro One more distro which is init based
Some users don't like to install systemd based distros because they increase booting and processing speed.
Pro Oldest surviving Linux distro
Besides Debian (which was first released in August 1993), it is the oldest still maintained Linux Distribution and was first released in July 1993.
Pro Super fast
Pro No systemd
Pro Simple packaging system
Built on Alpine's APKBUILD system, which is an accessible and simple packaging system. If you're familiar with Arch's ABS or Gentoo's ebuilds, APKBUILDs are a breeze.
Pro Is pure Python 3
There is a hard and fast rule against Python 2 software in the main repositories, with efforts focused on adapting software to use Python 3 where possible.
Pro Small and performant
A standard installation takes under 200 MB. Only the bare necessities are included.
Pro Inviting and receptive development team
The people building the distro are knowledgeable and helpful when issues arise. Merge requests are actively suggested and reviewed, and the developers thank users for taking the time to learn the distro.
Cons
Con Not user-friendly
It is made to be KISS (keep it simple, stupid), so you have to do everything by hand.
Con Large size live ISO
One needs to vain 3GB+ data for downloading one slackware ISO.
Con Narrow repos
Doesn't offer the same amount of options as other distros do.
Con Niche/small community
Slackware is its own niche and has a small community.
Con Very slow release cycle
Hasn't updated in 3+ years.
Con Dependency issues
When it comes to dependencies, Slackware shows more issues than many other distros.
Con Not (yet) ready for Linux newbies
As of November 2018, there isn't an installer yet. If you're familiar with installing Arch or Gentoo (via chroot, fdisk, et al) then it's no big deal. An installer framework (called Horizon) is in the works.
Con Is pure Python 3
Python 2 support is not supported by the distro, so many older upstreams who haven't adapted to Python 3 yet will need their software patched to work (this is both a pro and a con).
Con Somewhat limited package set for servers
As of November 2018, it's still missing some server software. It's primarily a desktop-oriented distribution, but accepts server packages and progress has already been made on that front, including lighttpd, apache, and php-fpm. Contributors are already bringing more server software to the distro, including certbot, cgit, and Nextcloud.