mid-drives are great, the gear advantage rocks; the bosch units suck but unfortunately they had so much industry sway that a lot of ebike frames are coming out built around the unit -- which is absolutely unlike any other unit out there aside from vague similarities to the bafang units in the same range -- so it forces replacement with the same garbage, or a total re-engineer of the gear train; a big pain.
one thing to be said about the bosch unit : it looks sleek and it integrates well. This is likely a lot of the reason behind the mass adoption, aside from Bosch's presence itself.