It violates the old school UNIX Philosophy of (1) simple standalone tools (2) text as the universal interface.

(1) systemd is a monolith that aspires to do almost everything from booting to networking to sound. It's in many ways a Kubernetes sans distributed systems.

(2) systemd uses binary logs, abandoning "text is the universal interface"

systemd is also a giant single point of failure.

Personally I don't see the big deal. The actual reason people hate systemd is that it's the type of design a software company like Microsoft would come up with rather than one that a bunch of gritty basement hackers would create. They have found refuge in OSes like Gentoo and OpenBSD, so systemd served as a kind of explusion of this type of programmer/user from the Linux ecosystem.