Acrylic, acetal and polycarbonate are each very different materials.
Acrylic is a lightweight, rigid and weather resistant Thermoplastic. Acrylic is safer than glass because of its greater breakage resistance, has a higher light transmittance than glass, and half the density of glass. In sheet form acrylic is supplied in a great variety of transparent, translucent and opaque colors. Acrylic, however, is not very scratch resistant. Acrylic does 'ring' nicely if you assemble a block or cantilever.
Delrin® is a homopolymer acetal for mechanical and electro-mechanical parts requiring extreme rigidity and dimensional stability. Glass filled 20% milled glass fibers by weight added to Delrin® base resin give Delrin® twice the stiffness of regular unreinforced Delrin® and impart a far higher resistance to elevated temperatures. Other additives to acetal can create different mechanical properties. I have used the glass-filled material, and believe that it would be a spectacular mouthpiece material, particularly for thin rails. Acoustically, acetal is a 'dead' material...it effectively damps out vibrations. Depending on your particular voodoo beliefs, this could be good or bad.
Polycarbonate, Lexan®, is a virtually unbreakable material with good dimensional stability and rigidity and a wide range of properties, including self-extinguishing properties. It also resist chemicals and provides high tensile strength...generally unaffected by greases, oils and acids. Lexan 'rings' very well. I have seen an all-weather marimba with polycarbonate tines. Again...believe your own voodoo about this one.