Could I use an sd card reader to update the bios and are there any benefits or downsides to it?
I've used an SD card to USB adapter many times and it looked (to the system) and worked identically to a USB flash drive. I've never tried to flash a BIOS with one but don't see why it wouldn't work.
The most likely (bad) thing to happen is the flash utility will simply refuse to identify the SD card as a drive to read from, the second worst is the BIOS flash utility refuse to read-in the file, the worst is once it reads it in it's corrupted and it refuses to start the update.
BIOS flashback is different...given it's finicky nature I wouldn't expect it to read-in the BIOS file at all. Flashback even refuses to work with many perfectly good USB flash drives.
The benefit to it is you can use that SD card you got for your digital camera you never use anymore. The downside is you've wasted your time trying if it doesn't work.