1. Does the part give clear "warning" that failure is imminent (e.g. sound, feel, appearance), or will it just fail suddenly? (This characteristic is often a key design feature of safety-critical equipment)
2. When the part fails (note, different failure modes should be considered separately), what is the range of outcomes? (E.g. drive failure is dangerous if you're crossing the street, not so dangerous at other times; bell failure could occur at the worst possible time, etc). Then on the other hand we have structural failure of the frame and handlebars etc which are almost guaranteed injury.
So if you are riding a bike with lightweight racing components that aren't designed with a "leak-before-break" philosophy, riding in mountainous terrain in a crowded peloton, then yes - pretty much any small deviation from normal could cause a massive pileup. On the other hand, a leisurely commmuter ride on a quiet path has much more tolerance for component failure.