Are all folding pedals a bit... bendy?
Yes, I've used a variety of folding pedals on my Dahon and Brompton and they're all a bit flexy. I've had plastic ones snap in half on me (and yes, that can be very painful/dangerous). The best ones I've had were all-metal ones in which the axle and folding pedal were both metal. But even those aren't as strong as fixed pedals -- and none of the folding designs are designed for putting all your body weight on (by standing to pedal).  This is the MKS FD-7, perhaps the solidest of the folding pedal designs, but also quite heavy:



Instead, the best pedals sold for folding bikes are the removable axle pedals made by MKS and others. Rather than the pedal folding, the pedal axle disengages from the crank with a quick release. This removes the folding component  and so this type is stronger and more rigid. The other advantage is that you can also get clipless as well as regular pedals. But even with this design, the pedal will still remain the weakest link in your power chain. The removable axle thins where it enters the crank (which means that the bearings are also smaller) and that the pedal retention system while improved and generally bulletproof is not foolproof (they keep making better fools).



Here is their regular style Ezy pedal:



The ideal design would involve cranks that had a custom pedal quick release built into them  but that would mean you wouldn't be able to use standard pedals anymore and would be locked into that manufacturer, which makes me wonder why Shimano hasn't done this. :)