The PHEV caliper issues are generally neither of those 2.
For the fronts it is a bit piston corrosion, but also sliding pin corrosion. Both of these issues are nothing to do with fluid used etc, but are all about them not getting exercised much. The way that we tend to try and drive these things for best range means that we use regen braking and easing off the gas pedal rather than hard stops on the brake pedal which move the pistons significantly and frequently.
For the rears, it's a bit of the same of the fronts, but with the additional problem of the parking brake mechanism corroding as it doesn't always get used enough due to lack of piston movement and owners just putting it in P (or just switching off without applying the parking brake which defaults to P and locks the transmission negating the perceived need for the parking brake).
Both of these issues can be avoided by better quality materials, but as the calipers are platform shared with French cars, that doesn't happen due to cost constraints on the suppliers. So, as owners we have to take care of them ourselves by protection/lubrication (regular cleaning and servicing) and regular exercising of the calipers through hard braking.
Basically, they don't make 'em like they used to!