Systemd is huge. It's a complex project that covers not just the init system, but also process management, networking, mounts, sessions, many other things. Many people think its monolithic design run counter with the Unix philosophy and wish to use distros without systemd.
Just to avoid misunderstandings: it's not a monotolithic blob, it is thought so because its first project was a system daemon that manages system services. It is described as "a software suite that provides an array of system components for Linux operating systems.", it is highly modular and offer many optional components that each have their own purpose.