Aligned hybrid-fibre discontinuous composites offer the ability to tailor their mechanical response through careful microstructural design. However, with tailorability comes microstructural complexity, which in turn leads to many sources of variability and defects. A virtual testing framework was further extended to investigate the influence of variability and defects on the mechanical performance of various aligned discontinuous composite material systems. This approach identified the most critical sources of variability as (i) fibre strength, (ii) the distance between fibre ends, or (iii) the level of fibre-type intermingling, depending on the material system. Fibre vacancy defects were shown to have the most significant influence on the strength and ductility of aligned discontinuous composites, although this sensitivity can be reduced through hybridisation of the fibre types.