Slackware's a bit of a peculiar case. It does indeed use /etc/rc.d in a somewhat-BSD-style manner, but there are two key differences:
* Slackware doesn't use a BSD-style /etc/rc.conf. Instead, daemons are enabled/disabled by setting the executable flag on each initscript. While this loses some of the features of /etc/rc.conf, I personally like this method better.
* Slackware doesn't include an equivalent to /etc/rc.d/rc.subr, which means there's a lot of boilerplate in its initscripts.
But yes, systemd's primary objections to shell-based init systems in general seem to stem from a limited scope.
In part part the reaction is to the boilerplate nature of sysv scripts, as they bring all their logic with them.
but if you glance outside the RH/Debian sphere you find the likes of Slackware, that has been using BSD style init for decades.