[Disclaimer: Yes I hate systemd and I proclaim that loudly. Everything below is my personal experience with systemd and why I hate it.]
If booting the machine up was all it did, then I probably wouldn't care. Most of my hate (I can't speak for the rest of the internet) comes from the fact that systemd does a lot more. It also tracks user logins using a mechanism (control groups) that isn't available in some container scenarios making systemd unusable in those environments (and by extension any distro mandating systemd). It does its own logging in binary which needs a tool to read the logs and if it gets corrupted then systemd's devs say "just delete the logs". Really?
But I think the best reason people hate it is because it makes other applications become dependent on it. GNOME is the most well known example but I've also seen that Centos7's Source RPMs have systemd-specific commands (macros?) making it hard to build them on other platforms. rsyslog doesn't listen on /dev/log because systemd is doing something with the socket now. You cannot start services without systemd being the one to do it.
This is the hate. systemd isn't just an init system, it becomes part of your daily life. I liken it to the MCP (Master Contrl Progam) from the first TRON movie. It's systemd's way or the highway.