I once did a test with a scrap piece of carbon steering tube and a stem having two large M6 clamp bolts. I tightened the bolts to a ridiculolusly high torque with a long handled hex wrench and it caused no damage whatsoever. I didn't use a torque wrench to get an exact torque value, but only an idiot would use a long handled wrench and pull as hard as he could to tighten a stem. That was the idea, to see if an idiot's approach to the job would crack the steering tube.

I expect that most damaged parts are either poorly made or have a sloppy fit with the mating part, causing pinching and a stress concentration.