Reliability in that I've used flash drives and SD cards for years but have only ever had issues with corrupt SD cards (probably at least half a dozen times) while I've never had any with flash drives.
Constant writes is an issue with them, which is why I think it's stupid that the Raspberry Pi Foundation continues to use them as the default storage/OS drive. Then again, they continue to make insane choices with power supplies as well, so it shouldn't be a big surprise.