Plastic rarely holds the pressure. It protects, supports, insulates, and houses.
Pipe and tubing ends ship with molded thread protectors so the threads arrive undamaged. Pipelines rest on molded spacers and clamps that keep steel off steel and stop corrosion where they touch. Instruments, meters, and transmitters sit in molded housings. Valves carry molded seats, handwheels, and position indicators.
What these parts share is exposure. Crude, condensate, drilling fluids, treatment chemicals, and sour gas all attack some plastics. The resin is chosen for the worst fluid the part will meet, not the average one.