If I recall correctly the Neumann KM 53 used an aluminum diaphragm rather than nickel, and the M 50 went through four different capsule types one of which had a nickel membrane, but it was used only very briefly--Neumann changed over to Mylar in very short order as soon as it was available and had proven its worth.
The main reason for the change from nickel to Mylar was reliability. A nickel membrane can be ruined in an instant by exposure to wind, or if some bozo blows into the microphone to see if it's "on." When the diaphragm "bottoms out" against the backplate, the polarization voltage causes electrical arcing, and that tears little holes in the material. By contrast if that happens to a Mylar diaphragm, you just remove the polarization voltage, the membrane "unsticks," and normally no harm has been done apart from the interruption of service.
Where metal diaphragms arguably have a slight technical advantage is in the manufacture of pressure transducers (omnidirectional capsules), since those are more highly tensioned physically than pressure gradient transducers (the resonant frequency is at the upper end of the frequency range or a bit beyond it), and it is somewhat easier to bring a metal diaphragm up to a constant, consistent high tension than it is with polyester film. However, that hardly matters nowadays.
Mylar film is a patented product of DuPont. Gefell was "behind the Iron Curtain" as we used to say, and lacked both hard (Western) currency and any legal way (or government permission) to draw up sales agreements with Western sources of supply, so they were forced upon their own ingenuity and locally-sourced materials. The fact that they continued over the years to manufacture PVC and metal diaphragms reflects the mixture of choice and necessity in their very particular set of circumstances; it wasn't purism for purism's sake.