Fermentation is a natural process, so there’s no energy input to the process. As for output, you get almost the same energy density as standard diesel, and some excellent fertilizer. For a farming area, it’s very self sustaining. Farmers bring their excess crop waste and manure, they get fuel and fertilizer in return. Also, the methane that off-gasses during fermentation is collected and burned to supplement the power grid.
Considering it’s mostly being used in this application, it works very well. However, I can see it falling way behind if scaled for widespread use in industrial and non-argricultural uses, where dedicated crops would be grown and harvested. Regardless, it’s a good way to cut down demand for standard fuel products for areas that can sustain a large biofuel digester.