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From what I remember from a materials class I took (10 years ago now so I might have the terms a bit mixed up)
Most materials have a fatique limit, if you stay below that amount of force there is no permanent deformation of the material. Plastic doesn't have that limit, which means any force applied causes at least some deformation.
Now with the plastic engine block, unless it has a metal structure that is then covered in plastic that the head, and bearings bolt to, the crank will slowly over time stretch down away from the head. The engine will slowly loose its compression ratio, and will start dumping oil out of the front and rear main seals as they will become ovals instead of circles.
Truly a throw away design.
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