Plastics with screws holding it will break at the screw holes. There is not much of a fix for that. Use only fast set epoxy or instant glue if you are trying to do some field repairs on plastic parts. Chemical bond glues, like model airplane type glues, will break down most of the plastics used in sights. Fiber optics that have a tight bend will break at that bend.
Bends create stress.
The first place the fiber will break is where there is stress. Exposed loops in fibers are just waiting to catch a limb or twig. All plastic ages with time. UV light and heat break down plastics. Plastic will get brittle and fibers will lose brightness.