Things looked great until I realized yuba has no disc brakes.
Disc brakes are standard. But if your rear hubmotor doesn't allow rear disc brakes (my motor didn't at the time - was a non-disc 5304 old school), then you will find the disc brake stress on the front forks a problem if you work them too hard http://www.endless-sphere.com/forums/viewtopic.php?f=3&t=40902&hilit=+yuba+disc
Otherwise, if your rear hubmotor allows discs, then you can get one working with some spacers and tweaks. It requires spacers because whilst the Yuba suggests (by virtue of the spacing of its disc tabs in the rear) that it wants a 203mm rotor, it doesn't (because this will rub with most hubs on the frame), it needs a 185mm rotor, but needs a random bizarre spacer to get it to work. Still don't understand how that came about in their design, I really don't, but have seen it working.