Generally I see a few:

• People wanting the highly deterministic, but slower behavior of the rc scripts.
• People liking the fact that the rc startup was generally almost entirely defined in plain script files
• Some folks criticizing certain opinionated things in systemd, as systemd delves deeper into things like capabilities and users.
• Systemd can sometimes be a bit weird about how it does/does not capture stdout/stderr as one might guess in some situations.
• Some folks not liking the journald angle of binary-only files

Mainly the last point is the only one I personally find potentially aggravating, but since I never really am in a broken system without journalctl I'm not too bothered by it. I have saved myself some effort thanks to systemd including stuff that the daemons used to provide for themselves.