Are piston ar15s more reliable than direct impingement?

They can be a little more forgiving of less frequent maintenance, but the leading cause of jams with AR pattern rifles is actually from the cheaply made mil-spec magazines, not the action. Also, with a piston system, carrier tilt becomes an issue of concern... they do make carriers and buffers designed to resist carrier tilt, though.

The average shooter probably won't actually gain anything from a gas piston AR vs. a direct impingement rifle, whereas, say, a Navy SEAL would, as they wouldn't be required to open the action and drain out the system after it was submerged before firing it.