I faced this problem many times, on booting/start or shutting down/stopping different services (iptables, NFS, ...).
[ **] A start job is running for Packet F…ramework (4d 30min 32s / no limit) I have some unbootable systems (like you see above) and some that hangs on shutting-down while waiting for stop job (requiring a hard reset) >From my point of view, despite being an issue on the related service not properly starting/stopping in timely fashion, is systemd responsability to either timeout and abort and continue the system boot process to be able to troubleshoot the service. But all the stuff I find on research (forums, google, ...) are specific problems/solutions for specific services. While systemd is waiting, I have no SSH or even console access (well, only to see the above message), so I cannot do anything even interactively. Note that some of the systems like Pi don't have a normal BIOS/Bootloader, so passing boot arguments in commandline is not always practical or even possible). As per the man page systemd-system.conf (5), the default is 90s, but as you can see on above line it says 'no limit'. Without looking for a flame-war, I don't remember this happened with old-init, when if a service failed it simply failed that unique service. Any ideas?
