A metal surface — raw, ground, anodised or plated — reflects light strongly. That makes lighting choice and setup matter more here than for matte plastics, and a reflection can be hard to tell apart from a scratch without proper optics qualification.
Defects on metal components — cracks, casting porosity, machining burrs, corrosion — can be subtle and require high resolution relative to part size. Assessing camera and optics against the minimum defect size that needs detecting is the first qualification step here.
Because of material weight and hardness, metal components also often have different positioning and fixturing requirements on the station, which affects imaging repeatability — and therefore inspection quality.