We got a particularly bad vegetable-chopping gimmicky gadget last Christmas. Sadly I can't remember what brand it was to warn you off it, as we threw it away[*]. It had a grid of blades in a circular frame, which you bring down at an angle onto a cutting surface, which has a grid of grooves matching the position of the blades. A bit like a mandolin but in 2D. It was explicitly designed for onions among other things. However, the very first time we pressed the blades down onto half an onion, several of them either bent or broke, and the onion merely got slightly squashed.

[*]Oh, wait, we still have the instruction leaflet. It was called a "Dicer +", no other brand name visible. Can't find it on Google. It looked very similar to this "JML Nicer Dicer" (http://www.jmldirect.com/uk/food-preparation/nicer-dicer/invt/n01nd20100000001/), but maybe that one is made of better quality materials.

Sounds like more or less the sort of thing gerald_duck is worried about above! I shall make sure 'must have Mohs hardness superior to even an above-average vegetable' is on my list of criteria :-)