Time Machine allows to back up to flash drives. Note, however, that flash drives are much slower than SSD drives, so both backup and restore with USB drives will be slow. For instance, it took 48 minutes to back up a disk with 12GB data to a flash drive and 67 minutes to back up 71GB to an external SSD drive. If you need a help with choosing a proper SSD drive for backups check the recommended external drives here.