I've seen copper fittings fail before thicker pipe does before. That was on a swimming pool. Too much chorline I would think.
Though if the water is good, it should last a long time. In Washington, I use type L underground.
For underground plastic, I don't see anything wrong with PVC, though like you mention, the fittings are the weak link there.
I have noticed lately that some Male Adapters look to have stronger looking thread portions.