Especially with bigger/higher cars now, if wouldn't be impossible to not see one of this carts on the back of a bike and i can't bear to think about what could happen.
Even in a normal height car, the trailer is often below the level of the door mirrors. I nearly took out the trailer of one of these years ago (it was the first time I'd ever seen one).
I was in a queue of traffic, waiting to turn left (and indicating), and could see a cyclist coming up on the inside. I crept forward slightly, and after the cyclist had passed started to turn. Thankfully, I was going incredibly slowly and spotted the trailer out of the corner of my eye just as I made the left turn, and I must have missed clipping the trailer by a matter of inches. It was completely invisible in my door mirror, and because the cyclist had been on the inside of me, it wasn't visible in my rear view mirror, either.
It still makes me feel sick when I think about it now.
I also wonder how healthy it is to have small children, whose lungs are still developing, at exhaust pipe level in heavy traffic. They must be breathing in a huge amount of CO.