Now I have to put in a word of defense my fellow German countrymen from Hepco & Becker, who are manufacuring those plastic boxes for KTM!
I used two setups so far, one grey set I bought for my F800GS and which I sold with that same bike, and one black set that came with my 990. No problems with any of those! I even crashed once, when I passed a tree too closely on a narrow trail. The left box came off (nice safety feature), I drove in the ditch, but nothing broke. Just open the lock and reattach the box.
However I can imagine that in hot sunny climate the plastic will age faster (UV radion is not good for most kinds of plastics) and something might crack/break after some years.
I sold those boxes again with the 990 last fall and now I am using soft luggage, which is much lighter, offers the same volume and also is totally watertight.