I have a ds640 (an usb scanner), and I've spent maybe 2 hours installing&reinstalling&removing drivers from the aur. Nothing worked. Installed Fedora just to see if things worked over there (as fedora is officially supported by brother) and things worked perfectly. Then i booted on my arch install arch once again, AND EVERYTHING WORKED. Alleluia. But I still don't know why, as I didn't touch nothing between last time i tried and now. Idk.
I can't even get this Brother to scan to a flash drive in its own USB port. It acts like it's successful; it scans and no errors show up... but the files just aren't there. Tried multiple USB drives and made sure they were formatted to FAT32 in a sector size that Brother recommended in the manual.
Printing to it from Debian was even easier than expected, though. Plug it in, it shows up as a networked printer, and you print to it.