The biggest problem people have with systemd is that it's constantly growing, taking on more functions and becoming a dependency of more software. People joke that some day you won't be using Linux anymore, but GNU/systemd, (or as they've taken to calling it, GNU plus systemd) because it's ever-growing from a simple init daemon into a significant percentage of an entire operating system.
People worry that some day, you won't be able to run a Linux system that's compatible with much of the software developed for Linux without using systemd. Whether that's a realistic worry or not I don't know, and I don't really have a horse in the systemd VS not-systemd race, but I can appreciate being worried that systemd might end up becoming a hard requirement for a Linux system in a way that nothing else really is - you can substitute GNOME for KDE, X11 for Wayland (or Mir, I guess), PulseAudio for PipeWire and most stuff will still work, so the idea that systemd could become as non-negotiable an element of a Linux system as the Linux kernel itself rubs people the wrong way, as it functionally makes Linux with systemd a different target platform entirely to Linux with another init daemon.