It’s not inherently bad, it “fails” the Unix Philosophy of “Do one thing and do it well” but since Linux’s kernel is:

• Unix-like, not Unix
• Fails this philosophy, as it does more than one thing but does all of it pretty well
• systemd is just a bundle of tools that do one thing and do it well under one package, like Linux’s kernel

It used to be a mess, but that’s solved. The biggest reason to avoid systemd is mainly user preference, not anything malicious. 90% of current distros use systemd as its easier for the maintainers and package programmers to build for the general than each package and each distro having their own methods of how to do an init system and other tasks.

How Debian and Arch and Gentoo and Slackware and other big distros worked was different, and the maintainers of those packages had to know “Debian’s way” and not a general way that most places accept. Systemd actually solved the Too Many Standards! issue.

I’ve never really seen a big argument against systemd, but maybe I’ve just not heard it.

The argument is basically that it does too much and as the motto of Unix was basically “make it do 1 thing and that very well”, systemd goes against that idea.

You might think it is silly because what is the issue with it doing many things. Arguably, it harms customization and adaptability, as you can’t run only 2/3 of systemd with 1/3 being replaced with that super specific optimisation for your specific use case. Additional, again arguably, it apparently makes it harder to make it secure as it has a bigger attack surface.