Do you count signals as inter-process communication? I'd argue they are, and the init system definitely needs to cleanly shut down the system (using signals, at least). What degree of inter-process communication is acceptable, then?
Traditional RC does a SIGTERM to everything, and then waits a bit and then does a SIGKILL to everything. This means that if the SIGTERM failed to be handled properly, progress is still made.
Weird as in 3 minutes after someone SSHd into my workstation, X11 restarted. Until I tracked it down to a default systemd configuration I thought I was going insane.
Beyond the default configuration, which is somewhat defensible as NIS is probably not a typical case, it really bothers me that if someone else's login process has issues, everybody's logins get killed.