IIRC, it goes against the OG Gnu/Linux philosophy of having multiple small tools that interconnect well together. Systemd is seen as monolithic and cumbersome by some/most(?).
If someone is more connected to this debate feel free to correct me!
That's pretty much it. Systemd goes against the philosophy of "do one thing, and do it well" by doing a whole lot of things and being integrated to an extent that makes it pretty much impossible to use only an arbitrary subset of its components while replacing the rest with alternatives. I understand where the critics are coming from, but I honestly don't care either way.
SystemD is an init system (it initializes the userspace) and takes care of managing services. Its quite an important piece of software. There are many reasons linux elitists hate it . I honestly don't care about it as long as it successfully boots my system which it does. In fact I like the syntax for systemctl...