Just rode the Transport+ (electric assist version) at Trek World. No load, but people were carrying other people
around on them. Handling is decent, very bike-like. I don't have any other cargo bike frame of reference, but it sure handles better than riding a tandem with no stoker. Electric assist feels like their other pedelec offerings, seemless, like you have a stoker kicking in real hard, or a real solid tailwind at lower settings.
According to Trek Pavement product manager, racks are rated 20lb up front, 100 on top of the rear rack, 50lbs on the side racks. I specifically asked about the axle placement stability issues and was assured that it wasn't a problem.
I liked it well enough that it will be the bike I EP this year, so once I get one, I'll report back on it.