they are flash memory with no battery needed, and should be very reliable storage.
Naturally chips have been known to die, and one assumes that more expensive chips might have tighter tolerances, but I have used flash memory devices in cameras and mp3 players for years with many, many write cycles and never had a problem yet. In contrast I've had hard disks and floppy disks let me down and been thankful for a rigourous backup regime which has never lost me any information yet - which I fully intend to continue!