Disadvantages of unlogged tables
Logging is in reference to Write-Ahead-Logs.
Essentially, everything gets written ahead of time to a log. That log can be played back at any point in time if there is a problem with database constancy (crash-safety). This means data is written twice, once to the log with representation in memory, and a later point in time flushed to the table's heap.
The log can also be shipped to a slave and can play a role in point-in-time (PITR) replication.
Check out the documents on the PostgreSQL implementation of Write-Ahead logging for more information.

If the system crashes (not shut down cleanly), the contents of unlogged tables will be deleted as stated in the manual:

However, they are not crash-safe: an unlogged table is automatically truncated after a crash or unclean shutdown