I find it easier to debug and edit shell scripts than wade through large amounts of C code (and possibly recompile) in the event of a problem that can't be fixed by a simple reconfiguration. I guess it depends on what you're used to.
Specifically a problem that couldn't be fixed by changing systemd unit files?
(I don't mean for this to sound in bad faith; I'm genuinely curious as a user of systemd)
General statement.
It doesn’t work with Juniper routers that many large ISP providers use.
So I resort to ISC DHCP and its flexible DHCP OPTIONS.
1. It's not relating to the service manager part of systemd. It sounds like a problem with systemd-networkd which is a component that is entirely optional and easily replaced even on a systemd-based system (as you admit yourself).
2. The original question was about bugs in systemd that require inspecting C code where the equivalent bug on a non-systemd system could be diagnosed by inspecting shell scripts. ISC DHCP is not written in shell script.