Provided the flash drive does not have bad sectors with data in them, and is not provisioned the same way SSD's are (and they most likely are not over-provisioned), its safe to use DBAN - although DBAN does overdo things (ie multiple passes when 1 would suffice).
FWIW, DBAN - with multiple passes - will do a fairly good job of wiping an SSD as well. (If your adversary is a government department or equivalently, funded, the only way to be fairly sure absolutely none of your data will be recoverable is to use FDE prior to putting any data on the drive, or - if you trust it - to use storage which has FDE built in to the drive).
I realize what I've written above may be controvertial - please raise concerns in comments before downvoting me - but to summarise my key contentions -
This idea of multiple passes came about many, many years ago from a guy called Peter Gutman (who I've met) - "The Gutmann Method", and was a mechanism to wipe data off any type of drive available at the time. Even Gutmann advised that you did not need to run all the passes - but which ones depended on the drives of the time (more then 20 years ago). Subsequently drives have increased in density to the point that its believed that 1 pass of zeros is sufficient to make it impossible to recover data, although there is a week argument for 1 pass of random or pseudorandom data. (For the sake of clarity, I'm assuming that meaningful data is 1 byte or more - the error rate when I last looked was something like 25% per BIT last I looked many years ago.
The reason why a single pass on an SSD is not adequate is because of wear levelling. Writing multiple passes of random data will defeat this mechanism.