Hey Adelle, you should post this under the 8fun/Bafang forum to get more replies or more accurate replies, I should say. The only problem I can for see is cabling issues. The battery can be mounted on the rear rack, allowing the bike to retain its folding capabilities, but that requires longer cables from the batter to the motor.
I don't own a bafang/8fun system yet, but I'm looking into getting one in the future. Good luck.