USB flash drives has limited number of write cycles usually from 2000 to 10000 depending on the device. However, there are no limitations on reading data from any of your flash drives. Flash drives uses number of technologies in order to prevent data loss caused by multiple write cycles. The most common is wear leveling, which makes widespread data failure much less likely. There are other features such as error correcting code that will add additional resistance against data loss. If you regularly overwrite the contents of your flash drive, you will wear out the drive faster. USB drives are not built for repetitive write cycles! You should never edit files directly on your flash drive. This was proven as a common mistake which greatly accelerate wear and cause unexpected data loss. Move files onto your computer’s hard drive before editing and never run an application directly from a USB drive!
On the other side, flash drives are much more resistant to physical shocks and extreme changes in temperature than portable hard drives. In many cases USB flash drives are able to receive some type of physical damage and still have data intact.
When you purchase a USB drive, you should read reviews as features vary from one manufacturer to the next, and your usage habits can drastically affect drive performance. Spending a little extra on a high-quality drive will drastically decrease your chances of a serious data loss event.