In particular I was bitten by
[ -x /bin/btrfs ] && btrfs device scan || emergency_shell
Since I don't use it I had blacklisted the btrfs module, causing btrfs device scan to fail.
It's very unfortunate when emergency_shell is triggered without a real cause, since my headless maching gets stuck until I can go to school to fix it (i.e. type "exit" at the shell).
I should point out that this particular server I switched from debian to void when I got fed up with systemd-mount doing exactly that thing... (blocking the boot).