The major problem is the requirements and the system dependencies it introduces on a core component. http://harmful.cat-v.org/software/dynamic-linking/

systemd introduces a major PITA for the OS packager by putting a lot of dependency on a lot of parts with "moving components" making it both more vulnerable as 1 and more likely to fail (it is not complex it is complicated).

Systemd also tends to become a system requirements for a lot of packages making it unsuppressable.

It makes the problem of dependency solving logically totally messy without justifying this cost by any real improvement.

It is introducing the DLL hell for the most crucial part of the system (PID 1).