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.