Steering problems aside, I would agree that if he expects to pedal that thing more than a few hundred yards pulling a trailer loaded to 350 pounds, he's gonna have some rather 'burning' legs and a back that won't straighten out for a week (he can always look for spare change around parking meters while he waits to stand up straight.)<g>
On the other hand, why not give this guy a "slight" break? In his video, he said, "This self designed cargo trailer has a capacity of 350 lbs., and was constructed out of aluminum," or some such thing. He didn't say the boxes contained 350 lbs, or that the trailer was loaded to capacity. This is obvious by the way he so cavalierly whips the tongue around and up to connect to the bike.
I'm not making fun of the guy, just saying that I think he was somewhat less than honest in what he implied.