The second major issue is it growing way out of the bounds of being just an init replacement, thereby violating the so-called Unix Philosophy of doing one-thing well. For a user app to do that is one thing, but for a core OS component to do that just smells way too much like forcing Linux to be more like Windows, which a lot of hard-core Linux users are ideologically opposed to.
The third major issue is that on most major Linux distros, users are not given an easy way to avoid using systemd, no matter how much they hate it or oppose it.
The fourth issue is the apparent arrogance of the people responsible for systemd, and their off-hand dismissal of mature and widely respected unix conventions.
All of this adds up to a storm of controversy that the systemd people mostly brought upon themselves. Had they just been more humble about their creation and waited until it was mature and well-tested and did all the wonderful things they claim it could and should do instead of stuffing what was widely seen as a broken-by-design piece of garbage down everybody's throat, maybe much of the Linux community wouldn't have been nearly so outraged by it.