First of all, Black powder in that large amount ignited by a fuse, or electrical device other than a blasting cap or simular device, does not ignite all at the same time. It is in fact a slow process. There is several milliseconds in the ignition process in which the early stages of the explosion process the pressure will rupture of the barrel, but not all out the powder at that point has been burned. This unburnt powder would have been scattered out away from the center of the explosion and would be ignited as it was scattered, causing a larger blast area.
This secondary burning of the excess powder does not add to the pressure of the explosion itself, but does add to the flash and burning factor of the explosion.
So as far as being an "effective" weapons that would depend upon what your purpose for the weapons deployment was. If you purpose was to inflict large casualities while the enemy was crossing a causeway or stuck in trench it would be very effective. Not so much to kill but to disable.