I never heard the Tesla CF sills cracking. I've put a good portion of my body weight pushing down on that sill without addressing the seam issue underneath without any stress or cracking of the sill. I'm a soul believer you get what you pay for, if what you pay for is a quality made product.
Also I've used CF / done CF work myself to strengthen up a stress area around a transom / floor area of my fishing boat due to the slamming of the waves and the weight of the outboard motor over time. Essentially the leverage of the outboard coupled with vibration across the floor caused cracking of the fiberglass where the vertical supports are perpendicular to the floor. I repaired it with CF, lasted for a little time, cracked. Did it again, adding another layer, again cracked. After I found the right amount of layering and the correct resin mix it never cracked again. CF is very very strong, as long as the resin mix is does not have too much catalyst as well as you lay enough CF cloth that covers the overhead of your stress points/area you're good to go.