Having studied materials at university, I would consider engineering polymers to generally be superior materials for lens barrel and camera construction to metals for most uses - and the 100 L macro certainly doesn't feel insubstantial to me. The only L lens I've owned that does feel lighter than I would expect, but is not flimsy, is the 70-200 f4 L IS, and the only L lens I've had that's fallen apart is my ancient and metal-barrelled 20-35 f2.8 L, and that only after it was dropped - and it used to belong to a press photographer in its prime, anyway, so it's had a hard life.