One big dif is that plastic fittings are cast in a mold and metal ones are cut with a die.
typically Plastic male fittings are screwed into metal female fittings.
Pipe threads are tapered, and male fitting are like a wedge.
The tighter you make the joint, the more outward pressure you exert on the female fitting.
A plastic female fitting on a metal male is likely to fail 1st?
The plastic male fitting will compress in the metal female
Plastic male fitting also can and do fail.
Their weakness is the valley of each thread.
They always crack at the thread outside the joint.
We try to always use Schedule 80 fittings.
If you have to use a plastic female you can increase longevity by putting a hose clamp around the fitting
