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Which would effectively mean it either being shit at low speeds (when you probably need it most) or inefficient at higher speeds (where it'll be used most of the time)?
Tijmen-
...ish, but electric motors (DC ones at least) are actually well suited to replacing human power, where you want maximum torque at low speed failing off as speed rises to deliver more or less constant power.
The main issue with hub motors is that they turn too slowly, being locked to wheel speed, to be optimum size and weight for the power required. BB motors can be optimised, but then the reduction gear takes up space and weight as well as cost, so it's far from a free lunch. As I said, good designs can be constructed around either option.
gbj_tester
That's the trade off, a hub motor has to work over the full range of wheel speeds, a BB motor only has to deal with the narrower range of crank speeds. Also, BB mount is better balanced if you have to pick the bike up, but that's not likely to be an issue for old people on cargo trikes.