Fuel in the fuselage reduces usable interior space, meaning the fuselage has to be bigger for the same cargo capacity. As do the beams for multiple wings. Multi

Fuel in the fuselage reduces usable interior space, meaning the fuselage has to be bigger for the same cargo capacity. As do the beams for multiple wings. Multiple wings along the fuselage put the after wings in the disturbed airflow from the forward wings. Many of the advantages they list would also apply to conventional designs, including fully-composite construction. The only advantage of multiple wings I can see is that the horizontal stabilizers on most planes generate downforce, which a multiple wing system could avoid. On the other hand, canard designs also avoid that and it seems the gains aren't so important as to get them adopted.