My older son is a Quad and he has had an electric assist attachment for his standard chair now for three years. It has worked wonders for him in that he can get out and about, biking had been a big part of his life before his accident, on wheels. His is a hand cycle though, Bafang BBS01 powered with an 8spd Nexus hub and serves the dual purpose of physical therapy. It is pretty much pavement only but that is also due to his intolerance for rough surface due to his condition. The first thing I thought of when I saw this was the lack of rear suspension which was brought up at the end of the segment.
There is a large scope of ability to those with either Para or Quad type injuries. Some can transfer as the girl does here but others are unable to do so as is the case with my son. The beauty of his system is that he can ride to a restaurant and undo the assist part and still be in his chair. But we have discussed a more off road type system with more shock absorption but haven't come up with a solution as of yet. This type of system wouldn't work for him due to the transfer issue and also how he would get the whole thing anywhere using his van. But a big thumbs up to the guy and gal shown for getting out there!