Performance, potentially reliability, and potentially longevity. They have slower read speeds and vastly slower write speeds, and may use lower quality flash. The exceptions would be several of the high-end flash drives, some of which use SSD controllers, but I do not think any of these are offered in a USB-C only version, and these tend to be physically larger, heavier, and they get extremely hot with sustained usage. If you are doing a lot of writes, anything except the fastest of flash drives gets annoying fast. (Obviously you want to back up the data regardless of if you are using a flash drive or SSD.)