what disadvantage? you could have tried it the last time you were in my workshop, it was the blue road bike on my stand. The motor weighs 2.65kgs, not much more than the crankset it replaces. There is no derailleur, no cable, nothing to adjust, not much exposes the kit to the elements, only an LCD to turn it on/off, set the assist level and speed limit.
The gospade has 120 tiny magnets on its pedal assist sensor. You don't have to turn the cranks much before the motor kicks in.
You can have a single speed bike, like the Gtech, which works and a lot of people are happy with it. You can have a 2-speed bike with the Automatix driven by a crank motor or you can have a front motor, like the Momentum. Some guys (not many it seems) bought Momentum bikes and were happy. You could put a three-speed SA gearbox in your bike, and maybe that would make you happy, but for most people somewhere around 7,8 or 9 speeds is about right depending on what type of terrain you ride over.
There's no exact number of gears that's perfect, but you normally know when you don't have enough. For most people, riding mixed terrain, anything less than 5 speeds is just not going to cut it unless you have a motor that's going to do all the work.
The justification for a crank drive is that you can keep it running efficiently by using the gears. The fewer gears you have, the more inefficient it becomes. The same applies to human beings. You have a natural power band. If you're pedal speed falls out of that, you lose efficiency. If you've ever ridden one of those Gtechs up a hill, you see the problem immediately. Without gears, you the motor and the battery are going to get tired very quickly.
Yes, when I was in my teens, I had a Sturmey Archer 4 speed, and I rode it happily everywhere in my ignorance. Does that make it good. Would I use one now? No, I think I'll stick with my 9-speed, which is many times better. I've converted a few bikes with SA 3-speeds. When I tried them, all I could think of was thank Christ I had a motor with decent torque.
Let's not fill ourselves with nostalgia. Compared with modern systems, they're just crappy. If they were any good, we'd all have them.
Mr Woosh, how many of your light-weight bikes with Automatix did you sell compared with say a Woosh Rio?