I read this answer to another question:
My indoor exercise cycle specifically cautions against using pedals which are not designated as being strong enough for indoor cycles. Supposedly, the stresses put on pedals on exercise bikes are greater than those on real bikes.
It is not apparent to me why a stationary bike would necessarily have to cope with more stress than a road bike. Assuming the same rider at the same cadence, the input force at the pedals would seem to be the same. So what might account for greater stress within the pedal or pedal attachment?
Granted, there are numerous other differences between a stationary bike & a road bike. Aside from probably very different construction, the stationary bike has to dissipate all the input energy somehow, probably as heat, which a road bike doesn't really do. But still, I don't see how this affects the pedals, specifically.
A possibility is the assumption that a road bike frame + tires absorbs some amount of the "shock" of riding. But pedals on a road bike also have to cope with bumps & other external shocks, so again its not obvious this would reduce the maximum stress they would experience.