If you think about it though, the number of cycles these parts go through is staggering and they actually last quite a long time so I'd say the material currently being used is bang on. (and I'm sure the mfg's tested ALL KINDS of different materials over the years)
Take for example pop bumpers. There are two pieces that attach the ring & rod assembly to the coil plunger, one is made of bakelite and one is metal. Pop bumpers only see a fraction of the cycles flippers do, and what part of the pops are most prone to failure? The metal yoke piece that attaches the ring & rod to the plunger.
Out of all of the games I've owned and serviced over the years I can honestly say the plastic plunger links are far from a weak design.
D