|Flash drives are the least reliable of modern storage media. They should be used primarily as a data interchange media, not as primary storage for important files.|
Wherever they are stored, important files need a backup copy. If they are of particular importance then you should have 2 or more backup copies, preferably stored in different locations. Having a single copy of important files is asking for trouble.
Even the very best of hard drives sometimes fail, often without warning or apparent cause. In some cases data cannot be recovered without expensive professional data recovery, or at all.