"Some people try to imply that the shell was a good debugger. Well, it isn't really. In systemd we provide you with actual debugging features instead. For example: interactive debugging, verbose tracing, the ability to mask any component during boot, and more."
suggests to me that, if you have some problem booting with systemd, you get a special systemd debugger rather than the shell. This makes me worried, because I suspect that I might have problems unrelated to systemd, and might actually have to run the configuration tool for some other service that's needed in order to authenticate users, and a systemd-specific debugger might not make that feasible. However, the truth is that the shell is a good debugger, and it's what systemd actually provides, along with command-line tools to help debug systemd in particular. This is a particularly unfortunate statement, because it plays into the impression people have of systemd that the developers think the tools you understand aren't good and you need to use their new thing instead in order to work with systemd.