Perhaps it is the idea that a linux machine should be wholly modular and attaching a library to a critical component of the system, shouldn't be a viable strategy for popularizing one's work.
When a distro is forced to carry a package due to a dependency of a dependency, or any magnitude there of, it breaks a core separation of power there. The users depend on distro's to provide reliable packages, however if a package is intentionally interweaving files to make these dependencies simply a part of the file and therefore robbing the distro's the ability to choose a different dependency should another developer or team thereof prove more reliable or more suited for their distro.
Systemd in this sense would be like microsoft robbing those wishing to distinguish themselves of the ability by increasing the magnitude of difficulty in doing so.
Now, keep in mind, I am not fluent in any programming language and have not audited Systemd, nor do I know anyone who has. This is based on a compiled understanding of observations expressed in arguments both infavor and against Systemd.