It seems obvious. If i spend 600€ on a new bike and 600€ on a conversion kit, i will get a very high spec e-bike. But i would have to spend double that, about 3000€ to get an okay quality off-the-shelf e-bike.
The off-the-shelf e-bike won’t be the exact size/style i like. They are nearly all city-bike style. Not many sizes are available, usually just small, medium, large (whatever that men’s).
It will be much heavier.
It will be more difficult to customise, for example with baby seat, lights, horns, mirrors.
It will probably be impossible to de-restrict, to be usable at normal cycling speeds.
Most importantly, it will not use standard parts. For example if the battery or motor dies after a few years, i have to buy a replacement from the same manufacturer, if that is even available. For a converted bike, i can change the components independently and choose any brand for the new parts (i think).
What is the argument for buying an off-the-shelf e-bike? Why would anyone do that?
Front wheel conversion was the most common last time I looked because you don’t have to mess withe the current drive train. In a hilly city front wheel drive loses traction on uphills especially if you put the weight of a kid on the back. I am talking about real steep hills - higher than 10% grade. No performance difference that I have heard of on flat areas.
I think it’s less efficient than the mid drive though - less power and less battery life - because the motor isn’t using the gears. Better to spend a few hours and buy a few tools, and get a better bike out of it.
With the front drive, I’d also be afraid of understeer in the wet, but i I’ve never heard that it’s a problem. The steepest hill on my route is 30%.