Of course your systemd startup can start a sh script at ease. I think systemd is going to make my life so much easier. Being more on the dev side of devops, but still needing to deploy correctly restarting aps depending on complex systems working. i.e. NFS mounts being available for data etc...
Slackware has always used BSD init scripts and it's one of the oldest distros. You used to be able to choose any init system you wanted but it seems that systemd is becoming entwined with "Linux" in some nasty ways. I wonder how long Slackware (or any other distro) will be able to avoid using it since software like Gnome is starting to require it.
FWIW, systemd can also call scripts under /etc/init.d if that is really what floats your boat. It is fully backwards compatible, you just lose a lot of the flexibility of systemd when doing that. The redis-server package in RHEL7 (from EPEL) installs an init script and I know that systemctl starts it.
systemd's declarative unit file syntax is easier to reason with, but comes at the expense of having to memorize a ton of options and being fundamentally dependent on the toolbox provided to you by systemd, since you can't code your way out of unconventional corner cases as easily.
The unit file syntax isn't the reason people complain, though.