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.
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.