1. If postgres shutsdowns uncleanly, your entire table is truncated; you lose everything.

2. You should check if your backup method backs up unlogged tables. For example, RDS Snapshots on AWS do not backup unlogged tables.

These 2 are a double whammy where if you aren't aware of these tradeoffs you can find that a bad restart has deleted all your data, plus your unlogged tables were never backed up.