Scope creep, violation of the Unix philosophy, large attack surface area, second system syndrome, interoperability concerns. It didn’t help that the creator’s other well-known project, PulseAudio, was also controversial at the time, for similar reasons.

PulseAudio, when it came out, was utterly broken. It was clearly written by someone with little experience in low-latency audio, and it was as if the only use case was bluetooth music streaming and nothing else. Systemd being from the same author made me heavily averse to it.

However, unlike PulseAudio, I've encountered few problems with systemd technically. I certainly dislike the scope creep and appreciate there are ideological differences and portability problems, but at least it works.