I have been monitoring backups to USB drives for three years after I noticed too many failures with client's drives. Monitoring the event log on the computer being backed up has been revealing. Even with backups that are successful, write errors are recorded in the event log. Subsequent disk checks of the USB drives revealed either bitmap errors or MBR errors.
Since any disk error will eventually lead to data corruption, it is a disaster waiting to happen. Even though the backup may be "successful" according to the backup program, write errors or disk errors count as a failure as you are playing russian roulette with your data. As Journeyman said, the point of backing up is to not have failures. Backing up a 99.999% reliable device with a 90% reliability device does not make sense. So I have opted for backup to NAS onsite and offsite backup through the internet.
The reasons for a high USB failure rate are actually quite simple, but not always possible to avoid.
Failure to remove drive through "Safely Remove Hardware" icon.
A full computer backup through software such as "Shadow Protect" (my favourite) push through large amounts of data at high speed. While most USB drives are sold with high data rates, the reality is that sustained high data rates create heat in the USB electronics. And since most USB drives do not have fan cooling, the electronics go over temp and stop until they cool down. Some drives are better but they all do it.
Again because of high data rates and no fan cooling the actual hard drives quickly go over the rated internal temperature, the temperature of which varies from drive to drive. And the higher the temperature the quicker the rate of temp rise. Hard drive life is shortened but an instant killer is immediately unplugging the drive while it is still hot.
The way people handle or abuse USB drives causes pre-mature failure.
The failure of some USB drives to properly dismount, so the client unplugs it anyway.
I have had to recover data from countless USB drives for clients, and the failure rate is too high. Several times, when client computers bit the dust, a restore from a backup USB drive also failed.
As per Ligos, taking the drive out of the USB case will help but this is not a commercial solution. Some drives still get piping hot even out of the case, and forced air cooling is necessary to get the drive under max rated temperature. I use HDSentinel to monitor temperature and also for general drive health checks.
There use to be small Verbatim portable drives that had a fan and connected by Ethernet but they are no longer available. For the moment it is backup to NAS and online backup for critical data.