There's basically three non-reasons to hate systemd which crop up the most: 1. Its lead author is a bit of an asshole, plus lingering resentment from Pulseaudio. 2. Systemd is not sysvinit, so some of the system administration people learned doesn't translate. 3. Systemd contains functionality that people don't think it should have (usually presented without any argumentation as to why it shouldn't incorporate those features, so it tends to come across as akin to saying that Firefox is bad because web browsers shouldn't have video players built in).

The main reason that does come up that's not really a non-reason (but, I should note, is hard for me to validate as a dispassionate neutral observer) is a fear that systemd is being rammed down users' throats against their wills, partly by distros deciding "there's no alternative" and partly by systemd incorporating ever more tools.