There is also a second disadvantage besides the extra weight of the bottom bracket;
usually these drives are operated via a Bowden cable.
If the drive is mounted in the bottom bracket instead of the rear hub, the cable has to go around two corners.
(The upper and and the rear corners of our triangles.)
It would not possible to use the front corner without a separable cable and a folding disadvantage…
The preferable drive technique will be this, I mean: