semilog
curmudgeonly optimist
The lens assembly is not plastic, it's made from Silumin, a silicon-aluminium press powder material. It has no advantages other than cost in mass production. The material is very fragile, and the lens hood effectively attaches to it. Any stress (e.g. from a bump) is transferred to this part and it doesn't takes much to crack it.
tl;dr made from cheap sh*t with poor mechanical engineering
So far as I know, silumin is a somewhat generic name for a family of melt-alloys and sintered metal matrix composites. Some of these have quite good strength-to-weight ratios and toughness; similar alloys have been widely used in aerospace applications. Many such materials are anything but "cheap sh*t."
Depending on the specific alloy, manufacturing process, mechanical design, and finish treatment, a silumin part could be strong or weak. Just as with nearly any other material. The devil is in the details. Is there evidence that fracturing of the optical cell is a common failure mode for v. IV Summicrons? Is there evidence that failure is more common for v. IV vs. other Summicron generations?