I had a system which failed to boot. The last lines printed to the screen, which were not relevant to the problem, were:
[19.957783] async_tx: api initialized (async) [20.899732] Btrfs loaded, crc32c=crc32c-intel
Then a little bit later:
[32.469926] random: crng init done [32.470551] random: 7 urandom warning(s) missed due to ratelimiting
The full problem description is here. And the solution is here (careful about the typo on that page, the only relevant setting is GRUB_CMDLINE_LINUX_DEFAULT).
Basically the problem was a bogus GRUB_CMDLINE_LINUX_DEFAULT setting in /etc/default/grub applied by my salt config. I fixed the setting which fixed the issue.