Diaphragm first stages are an upstream design, so theoretically if the mainspring failed/broke it could fail closed as the tank pressure is gping to hold the seat and seat carrier against the orifice.
If the same failure occurs on a balanced piston reg, the piston is downstream so it will just sort of freefloat and operate, but with a fairly low IP, so it will breathe, but not all that well.
That said, mainspring failures are all but unheard of. In terms fo the likely to be encountered failures (Hp seat leaks, o-ring leaks, etc) they will all fail open.
Even on an unsealed diaphragm first stage, the moving parts are behind the diaphragm so they are not exposed to sea water during a normal dive. On the other hand, there are more smaller moving parts and springs inside that can be affected by water entering the reg through the air inlet as is the case with a tank with water in it or during a rinse with a leaking or missing inlet cap.
A balanced piston reg does have piston stem and the back side of the piston head exposed to water during the dive, but as stated above the parts are normally covered with a film of christolube. They are also chrome plated brass (or in some cases the piston head is plastic) so with reasonable rinsing it is just not an issue. I regularly see Mk 5's with 30 years of salt water service that are still corrosion free. If flooded internally, there are fewer moving parts and no small parts so corrosion damage is less immediately problem causing - but unless it is cleaned and dried soon, it is still going to encounter serious corrosion problems.
In short, there are pros and cons, but both designs are more than adequate and are durable enough to survive all but the most accomplished idiots.
