Disclaimer: I use systemd distros. I dont hate systemd, I just like the ability for alternatives to flourish without fighting an uphill battle.

It has major project scope creep (does too many things that arent init or service management), isn't modular or portable, only just gained support for muslc, it runs most of its init and management things in pid1 (which is a security and stability issue), it is a massive C program (large attack surface), it isnt very fast when compared to any other init (especially s6 or dinit which boot in under 4 seconds), it implements non-standard interfaces which just encourages further dependency, etc.

Systemd is like the Walmart of Linux OS tools. It replaces many other options and does things good enough (not the best, good enough) to make it worth it to use them and their ecosystem, and they make things simple to use. But just like Walmart, they undercut other options, stifle adoption, until they are the only shop in town.

Dinit does everything I need out of service manager, has similar command utilities and syntax to systemd, is much faster, simpler and cleaner code, avoids many of the pitfalls of systemd, supports user services. s6 is pretty good to but kinda terrible UX.

The simplest answer to why I dislike systemd is that with all the major distros using systemd, it will become harder and harder to use most Linux software without systemd and its growing set of utilities. If systemd made an effort to work with the community to implement standard interfaces then alternatives could flourish without requiring large on-going patches to much of the Linux software ecosystem. It will only get worse from here. Systemd is (basically) the init of Linux and I think that is sad.