Interesting. I was just considering switching to flash drives. But my concern is longevity. They are only rated to last for 10 years before bitrot can set it and ruin the photos. Currently, there are no "archival-grade" usb drives. DVD-R discs are only rated for about 5 years, but at least there are archival grade discs too, like the Taiyo Yuden TYG03 rated for 75 years, or the Delkin 24K gold series which is rated for 100 years.
This is making me want to stick to discs. The LAST thing I want is clients from 10 years ago contacting me for another copy of their wedding photos. At least if I give clients an archival disc that will hold data for the remainder of their life here on earth, it's up to them to transfer the data to current media.
But I am still quite attracted to customized flash drives. It would be a lot easier than burning a disc and preparing a print for the disc surface.