Historically, there were many reasons people disliked systemd, which are mostly irrelevant at this point.
- systemd was a massive departure from how init and logging systems worked at the time. It meant a lot of re-learning for long-time Linux users and some people dislike change. At this point, it is ubiquitous so most people have learned how to use it.
- People feared that systemd would homogenize Linux since most Linux distros of the time had init differences. In point of fact, this did happen. However, whether this is a good thing or a bad thing is matter of debate. Figuring out how to do service management used to require reading the documentation for each distro since it was done totally differently. Sometimes it involved running a command, creating symlinks or writing scripts. It was always an adventure. Now you can switch to any large mainstream distro and know how to get around since they are all using the same framework.
- systemd logging moved from simple flat files to more complicated structures that people used to dealing with flat files were not comfortable with. At this point, I think it is clear that systemd logging has more advantages than disadvantages.
- In the early days systemd was plagued by serious bugs which fueled the fire even more. At this point, systemd is much more stable.
There are, of course, some more legitimate concerns that remain
- Some people fear that since the code base is very large and complicated it would be easy to hide malware or a backdoor.
- There is a philosophical issue that unix is all about the creation of small, simple utilities that do a single job well and systemd is the opposite of that. In reality though, especially on the desktop, there are quite a lot of tools that no longer use that philosophy.
- systemd is much more than just an init system and it is increasingly encroaching on more and more elements of the system. This is making Linux increasingly reliant on systemd which decreases optionality to some degree.
For me, personally, I don’t miss the pre-systemd days. As someone who uses lots of different distros, having a common framework for management is a bigger benefit than the trade-offs.