When reviewing the different criteria, it appears that metallic parts may present less restrictions on dimensions, and so may be more affordable.
It is true. It is true also that lots of parts produced by thermoplastics high precision injection were first based on metallic parts. This means that while it is technically harder to substitute high performance plastics to metal parts, there are in fact important assets. Typically, substituting critical parts made of metal by thermoplastics materials allow gains of weight, of efficiency (for moving parts) or of noise and vibration (NVH: noise vibration and Harshness). With composites and high performance plastics, you may even gain in mechanical properties or in tribology.
One typical example for such gain are gears. Standard gears in thermoplastics are common. But when it comes to high precision gears, metal gears were the most common solution. With the development of high precision injection however, high precision thermoplastics gears achieve better efficiency, low torque and a clear reduction of noise, with no lifetime reduction.
Moreover, if you fear for lifetime, mechanical resistance or thermal dilatation, high precision injection is totally compliant with metallic insert overmolding. This solutions solutions combines the assets of metallic and thermoplastic parts on different functional surfaces, for a global gain on the final product.
So substituting metal with thermoplastics for critical parts can greatly increase the added value of the final product, and thus, overcomes the challenges of high precision injection.