We have chocolate milk in our lakes and rivers, usually can't see more than a couple inches below the surface. So I have run into many shallow areas and even a few submerged trees, once while moving at a pretty good clip, and have never had anything break on a Mirage drive. The one time I was moving fast, the tree grabbed the fins (or maybe the other way around) bringing me to a sudden full stop. All that happened was the front fin was bent back so far it overlapped the rear one. I had to bend it back straight enough not to overlap while sitting on the water - quite a feat, but I managed it - then just plugged it back in and limped back to shore. I was still able to use the drive, just the blade shafts were angled to the sides a bit so I couldn't get a full stroke. I straightened everything out once I was back home.
Considering the forces the blades and drive must experience when hitting shallows or submerged stuff, I can't see "mere humans" pedaling causing a breakage unless they're doing something like "hitting the stops" - hammering the pedals against the front or back end-of-travel. If the pedals are adjusted properly you shouldn't hit either extreme at all.
Though I did wonder about my nephews the other day. Whether I'm pedaling easy or hard, I tend to be very smooth and even over the entire stroke, same as I do when riding my bicycle long distances. My nephews, though, felt more like someone standing on the pedals and sprinting up a hill! Very short, sharp bursts of power at the start of each stroke. I suppose that sort of pedaling could fatigue things more quickly.