As I have understood, systemd is an init system that tends to integrate a lot
of functionality, while also being quite feature-rich. People criticize that it
swallows programs which were standalone before, and the tendency
of some programs to contain systemd-specific code, which makes them less
portable and forces the user into systemd.

Of course, there are binary logging formats, which I tend to find rather
meh.

I have had some negative experience with systemd, but I haven't had the
chance to compare it to any other init system.

http://suckless.org/sucks/systemd should give you a quick start if you want to learn more
about technical problems with systemd.