I am a double leg + one arm amputee using folding chair with one arm dual rim drive. My experience might be relevant for others who have less-than-para strength.
I had a loan of the SD for a week and think I'll get one but as the battery doesn't fit attached under the seat I need a solution to attach there. The battery was hung on the backrest push handles and the weight behind me is a problem and I think it puts undue stress on the backrest poles and the folding hinges. A friend has a fixed frame and the crossbar stops the battery fitting so the same solution would be necessary for him.
Also, as a fail safe for a button malfunction is to pull the battery cable, this is not possible with battery behind.
Features that I think are necessary for the device to give optimal assistance:
1. A brake function. The added weight made it difficult to control the chair on steep declines (motor off of course). Hills I would sail down confidently and safely became hard on my hand and arm.
2. A button press to boost the speed increment. On a steeper hill I could not give enough push to kick it up.
3. A sensor to identify the change from up to downhill in outdoor mode, and slows or shuts off the motor. Pressing the button to halt outdoor mode has to be timed precisely at a sharp change. An example is a steep rise before a kerb ramp ... I stopped the drive before the top of the rise then couldn't push to get it going again. As in #2 a button press to give a short boost would have worked.
4. A less sensitive indoor mode. In an open space that has changes in gradient or surface like a ramp or hard to thick carpet it sensed as braking so stops the motor. Also holding the push rim to change direction stops the motor.
5. A constant speed mode. This is for example crossing a road. Going down a kerb ramp kicks up the speed to more than necessary and safe but the drive is needed for the rise of a cambered road. Then the fall to a kerb ramp needed button presses.
Some or all of these might be in future releases of the drive. I'll be paying attention to developments.