I'll take the well documented (man launchd.plist) XML property list (well, XML rendered, they're usually in binary) any day over some flat unstructured nonsense. I loathe INI syntax.
As for ini vs xml, I've generally found xml is a crueller syntax for humans than ini. At the time I started using systemd, it was a bit funny - the last time I'd been editing ini files was on Windows 3.11. But I think ini and toml are now once again reasonably common so I forgot about how out of place it felt at the time.