Linkage looks bad for a product designed to push 150lb+ bike and rider.
Wouldn't be surprised if it might have a high failure rate... or owners just mis-installing it.

There's also no option to use this trailer for cargo, groceries, etc.

I would suggest a single wheel cargo trailer with a hubmotor wheel kit installed instead.
Check the size of the rear wheels in the available trailers and find a hubmotor kit that matches and you're done. Nashbar and Performance have some good prices on cargo trailers. I think they are 16" and 20" wheels respectively:

http://www.nashbar.com/bikes/Product...2_171080_-1___