The nice thing about initscripts, though, is that you can get the best of both worlds by sourcing in common functions, as is the default and recommended practice on OpenBSD. For example, here's the initscript for dovecot on my (OpenBSD-running) mail server (with some comments and newlines omitted for brevity):
Pretty straightforward: you set $daemon to the path of your daemon's executable (in this case, dovecot), source in rc.subr, and call rc.subr's rc_cmd command with $1 (which represents a command, like 'start', 'stop', 'restart', etc.).
rc.subr, meanwhile, holds all the complexity one will ever likely need in an initscript, in the form of various shell functions encapsulating things like starting, stopping, etc. And if that's not enough for whatever reason, it's easy to add more functionality either to the individual initscript or to rc.subr, since it's shell scripts all the way down. In my experience, reasoning about this sucks a lot less than writing a systemd unit file, let alone an initscript on a system without such niceties.
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.
rc.subr, meanwhile, holds all the complexity one will ever likely need in an initscript, in the form of various shell functions encapsulating things like starting, stopping, etc. And if that's not enough for whatever reason, it's easy to add more functionality either to the individual initscript or to rc.subr, since it's shell scripts all the way down. In my experience, reasoning about this sucks a lot less than writing a systemd unit file, let alone an initscript on a system without such niceties.