Wow what an old thread.
Disclaimer: I have no idea how valid any of what I'm about to say is. It is mostly just based on my limited observation and even more limited knowledge of the subject. It may be utter nonsense.
I've been running full install pfSense on USB flash drives for a number of years. With RAM disk enabled, and backup schedule for RRD at 9 hours, and DHCP leases 7 hours.
The USB flash drives seem to last about a year before starting to go bad.
They seem to start exhibiting file corruption. Though one went sudden total failure (probably a critical block went bad). Inaccessible/Undetectable. Could even detect and use it in a PC.
Not sure if the wear-leveling of USB flash drives is effective and able to detect and mark bad blocks within the active file system.
Two possibility mitigation techniques.
When a file is noticed to be corrupted. Don't delete it or edit it. Rename it. Then restore a good copy of the file. This may keep the bad blocks out of service so they don't become available for other write cycles.
Re-install to recreate the file system from scratch so the bad blocks can maybe be detected and marked.
Bare minimum fail safe: Have handy, 1) installation media, 2) a good USB flash drive, 3) recent config backup.