I think, based on the other comments here, and my experience with my own Brother printers... I think they are the "less clever" people building reliable printers.
- ethernet
- BR/Script3 (their PostScript clone)
- duplex
This advice has been good for the last 12 years or so. I never install their drivers; I just tell CUPS that there's a Brother in PS mode with a duplexer at a particular IP address, and then there's no further issue.