I've sold a good amount of black walnut as firewood (limb wood, not highly valuable trunk wood
). It has a BTU rating close to white ash, and well above green ash & hackleberry of which people are very accepting, yet I've fielded comments similar to yours on black walnut. I kind of chalked it up to it doesn't stack tightly in storage (less pounds per cubic foot in the rack), or in the stove and has very thick (non btu laden) bark, characteristics which would mean shorter burn times. Does it really not produce heat? Have never burned it myself.
I've been told it has a beautiful greenish flame by fireplace users buring for ambiance rather than for heat. Maybe that's a better use.