The biggest problem people have with systemd is that it’s constantly growing, taking on more functions and becoming a dependency of more software (for example, the Snap packaging system basically requires it). 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 (I’m using a systemd-based distro but that wasn’t a factor in my decision), 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.
There’s even rumours that the next version of Windows is going to inject a bunch of AI buzzword stuff into the operating system. Like, how is that going to make the user experience any more intuitive? Sounds like you’re just going to have to fight an overconfident ChatGPT wannabe that thinks it knows what you want to do better than you do, every time you try opening a program or saving a document.