The crux of the issue, at least as I understand it, is that traditionally you’d build the system components in a way that they were interchangeable by defining some standard-ish API. While you can interchange some Systemd components, since “ststemd-xyz” is just the umbrella name of the project as a whole, so some components are completely independent, the core ones can’t really easily be swapped, and it’s only getting worse, especially as systemd is starting to become a hard requirement, a lot of people are rightfully feeling peeved. There’s a great talk made by a BSD developer that admits the virtues of Systemd while bringing the downsides, so no blind hate, you can google for the video “The tragedy of systemd”.