It's not a "Registry for Linux" as many articles/posts have claimed,
It's something a lot more sinister than a "registry". Something like a registry could be added if one were really needed without everything that systemd is bringing into being.
it streamlines bringing up a Linux box and does away with a confusing script-based system
There was nothing confusing about it. When comparing the various sysvinit machines with my current crop of systemd systems, the way in which systemd is gobbling up various modules and making them difficult, sometimes impossible, to access is, if anything, overcomplicating what was previously trivial to work on.
that was years out of date and difficult to optimise/parallelise.
The age of the system is immaterial if it works. As for optimisation, sysvinit was pretty good where that was concerned and "parallelisation" is merely yet another fad which has little real use.
Having said that, the most recent Devuan beta felt like a bit of a let down when I tried it; even the first beta worked better for me. I'll give it a go with a view to possible future use but it all depends on how well it is set up and, more importantly, how well updating works.