It’s a giant mess of interconnected programs that could theoretically still be disentangled, but in practice never are. It was very quickly and exclusively adopted by pretty much every major distro in a short period of time, functionally killing off any alternatives despite a lot of people objecting. Also, its creator was already pretty divisive even before systemd, and the way systemd was adopted kinda turned that into a creepy hate cult targeted at him.
There’s nothing actually wrong with systemd. I personally wish there was still more support for the alternatives though. Systemd does way more than I need it to, and I just enjoy having a computer that only does what I want.