Agro-industrial residues could be used for solid-state fermentation (SSF) to produce enzymes, organic acids, biosurfactants, and microbial biomass; however, the results on the performance of the process have been varied and contradictory due to the differences in composition, physical structure, water relations and inhibitor content of the agro-industrial residues, and to the fact that this is seldom reported in comparable terms. The critical narrative review is organized around a single line of argument, which runs from substrate properties to microbial response, and from response to the formation of products, selectivity and reproducibility. A narrative analysis of publishers' records was conducted from August to October 2026 and sources were identified, with particular focus on those published between 2020 and 2026; a total of 28 sources were identified and retained that had confirmed bibliographic data. The evidence suggests that composition will determine what products are likely and the physical structure, water availability and sugar accessibility will determine whether the fermentation will reach them. Released nutrients in the process of pretreatment, but may remove the inducers, create inhibitors or cost; in one study, the reducing sugar increased threefold while the lipase activity decreased 11-fold. Units in the various studies vary, moisture basis and dry-matter basis are not always reported, strain identification is limited, and few pilot-scale, techno-economic and safety data were reported. It is proposed to build a qualitative framework that connects the three main product attributes (biochemicals, bioenergy, and bioextraction), the compatibility of the product with the targeted microorganism, the substrate, the conditioning, the process constraints, the safety, and the feasibility of scale-up, and the main gaps that can be expressed as research priorities to be tested. The framework is based on the published evidence and has not been tested in experiments.