The issue with diaphragm reg designs is that you have a large diaphragm that can become a potential fuel source.
In comparison a piston reg only has a few o-rings to feed a fire. EPDM or viton o-rings are more resistant than nitrile o-rings to burning, but they will all burn in a high PPO2 environment if the ignition source is sufficient.
Personally, I prefer unbalanced piston first stages for high O2 deco mixes as the design is very ammenable to high O2 applications. There are only 2 dynamic o-rings and both of them are in the intermediate pressure areas of the regulator. In addition, the only areas exposed to high pressure gas are upstream of the orifice, and unlike a diaphragm design, there is nothing upstream of the orifice except the filter and the tank valve.
In contrast, with a diaphragm design the seat and seat carrier and one dynamic o-ring and a couple of springs are all upstream from the orifice and live at tank pressure all of the time exposed to much higher partial pressures of oxygen.