I think you are referring to a swaged-on spider slipping with respect to the cranks. This can be dangerous, and it can happen with any cheap cottered crankset, but it is probably far more likely on a cheap cotterless set. Listen for creaks and groans, and perioidically check the integrity of the crank-to-spider junction. Better yet, find an Agrati crankset with an integral spider.

I have owned several bikes with cottered cranks, and the only failure I ever experienced was a left crank fracture at the spindle/cotter eye during a strong downstroke. My left foot lurched downward about an inch or so, but I maintained my balance and rode home with my cranks in a 160-degree / 200-degree orientation, i.e., with the left crank timing advanced by about 20 degrees. The outcome was far better than when I snapped an aluminum crank at the pedal eye during an out-of-saddle climb ...

As regular readers know, my red-with-white Capo has updated components, but I am keeping its white-with-red stablemate as original as possible, including the cottered crankset and balky Campagnolo Gran Sport derailleurs.