Wrote about present day reasons to dislike systemd a few days ago on HN, which encompasses most arguments of actual substance[0] (tldr: Unix philosophy, it homogenizes distros and it may be too heavy for some low-resource environments).

Historic reasons mostly come down to systemds developers being abrasive jerks to people. Systemd has some weird behavior choices that only really make sense from the perspective where every computer is a desktop; ie. it terminates all processes spawned by a user when logging out unless they were made in a specific way with systemd-run. This makes sense on a desktop - users log out, you want everything they did to be cleaned up. On a server it makes less sense, since you probably want a tmux/screen session to keep running when you sign out of your ssh session (either by choice as a monitoring tool, or alternatively because you have an unstable connection and need a persistent shell).

Every downstream distro got surprised by this change[1] and nowadays just ships a default configuration that turns it off, because upstream systemd developers weren't interested in hearing the complaints.

Most of these footguns have been ironed out over the years though.

There's also some really dumb arguments to dislike systemd, most of which just can be summarized as "people have an axe to grind with Lennart Poettering for some reason".

[0]: https://news.ycombinator.com/item?id=46794562

[1]: It was always available, but suddenly turned on by default in an update.