They are a good brand but basically just the same as any other pen drive, nothing special about SanDisk ones. They probably last longer than cheaper ones but they will still fail eventually without warning. Just don't use a pen drive if you want reliable, long-term storage. Use optical discs or external hard drives. I've had three WD external hard drives for over 10 years and they are all still working fine.
Only time I use a pen drive is to quickly transfer some files from one PC to another ---- I never store any data on it for back up purposes, too risky.