I'm an old school unixhead who's nonetheless made his peace with systemd because, honestly, it makes lots of things work way better than most previously available solutions did.
Thing is, it also forcibly changed a bunch of things by introducing defaults that were not at all what people expected. They might have been better overall, but it still caused some nasty surprises.
Let me try and explain a bit more concretely -
An easy example (not a hypothetical one, btw): if you have a physical server with multiple hard drives and one of them doesn't start, old school unix would boot everything it -could- boot anyway. SystemD changed that behaviour to "if a filesystem doesn't come up, and you don't set an option to tell systemd it's ok if that doesn't happen, don't finish booting".
Now, imagine you're somebody who has a physical server somewhere that has a big cache disk that you use a cheap drive for because if it fails, well, you've lost some cache space. Now imagine your server used to run a pre-systemd setup, and when you upgraded it to a systemd using version of the same distro you left all your settings as-is because everything seemed to be working fine. Now ... imagine that drive dies during a reboot ... and so the server doesn't finish booting, and so you can't even ssh in to it to figure out what's going on, and the only way to figure it out is to get physically in front of the console.
If that server is four hours' drive away, you might not be very impressed.
I still, overall, think systemd is pretty nice. But while some of the people annoyed with it are annoyed with it on a purely aesthetic/principle sort of basis, there are definitely some decisions the authors made that caused -very- surprising results for people who had been running servers for years already, and some of the dislike being thrown around is very much understandable.