Systemd invents its own configuration language (it looks like ini but there no standard for that and systemd’s flavor is its own) so you still need to learn it.

Yeah, but it's much more straightforward and less exotic than scheme. But I guess this type of configuration fits perfectly in Guix, where everything is already configured similarly.

Yes but as a somewhat layperson (electronics engineer and light firmware design, and some hobby sysadmin stuff), I can learn systemd's "language" in 30 minutes and most attributes are so self evident that you can puzzle them together without learning the language at all.

That Shepherd mess I would have no idea what to change to make a small tweak without spending hours and hours learning it because it is written extremely cryptically in comparison.

It's the difference between modifying a config with human readable names and having to go into the source code to change heavily abbreviated variables that require a lot of background knowledge to even read.