Agro-industrial residues can be used as fibers, particles, ash, biochar or cellulose-rich reinforcements in polymer composites. Their local availability and low density motivate lightweight products, but a residue-based formulation is not necessarily structural, biodegradable or inherently environmentally preferable. This critical narrative review is extended to consider representative feedstock families, matrix choices, preparation and forming routes, interface engineering, mechanical response, candidate structural applications, durability and recent advances through 2026. The evidence indicates that strength and stiffness are dictated by fiber geometry and orientation, matrix wet out, porosity, formulation and conditioning: treatment can assist load transfer but may also damage fibers or impose processing burdens. Consequently, results from different formulations cannot be collapsed to a universal property range. Illustrative studies highlight both opportunities and limitations: a surface treatment that cleaned kenaf did not necessarily strengthen its epoxy composite, whereas one FDM study observed smaller mean strength losses at 10wt% filler and greater losses at 20wt%, with trade-offs in impact strength. Moisture, creep, fatigue, fire performance, feedstock variation and end of life pose critical design constraints. Hybridization, bio-based matrices, multiscale reinforcement and additive manufacturing expand the design space. This review concludes that near term deployment is best in noncritical and semi structural products; primary load bearing requires statistically sound properties, service life data, structural validation and performance based life cycle comparison.

Keywords - agro-industrial waste, biocomposites, natural fibers, structural materials, mechanical properties, interface engineering, durability, life-cycle assessment, additive manufacturing.