How great is the risk of a pedal suddenly coming loose due to a hairline crack in the thread?

Even if such a crack were to appear in the pedal thread after many years of use, it does not mean that the pedal would suddenly fall off. If a hairline crack were to occur, it could cause the pedal to loosen in the thread, begin to wobble, and then wobble more and more until riding becomes unpleasant. At that point, at the latest, the defect would be apparent.

Of course, any mechanical device – and a woom bike is no exception – has a maximum service life after which signs of wear and tear may appear. 

Just like any bike, woom bikes need regular maintenance – brakes need to be checked, bolts need to be tightened, tyres need to be pumped up, and ideally every bike, especially one bought used, should be serviced once a year.