But what would be the actual result for a helmet that didn't work? Suppose it was as brittle as crystal and exploded at a very tiny contact, or it was made of cardboard and simply got torn up without offering any protection.

Either way you are left with a damaged helmet and a beat up rider that survived the accident, which is the same result you get with a helmet that does work. (Obviously if you didn't survive the accident you aren't around to post about it). If the rider doesn't sustain head injuries, then that can be attributed to the damaged helmet. If the rider does sustain some head injuries, then their mitigation can be attributed to the helmet with the alternative surely being death.

So my point is that posting about a helmet that worked in an accident is kind of redundant, because it is the only possible result, which is great because it means that helmets will always work.