risk/reward of carbon vs stock s rims is silly, as is the cost as are the negligible at best weight savings... in terms of materials and physics, the science says carbon rims are a riskier proposition; for example, on a pothole an s rim can bend and deform quite badly, but a carbon rim can literally explode in fibers and epoxy.
you can have 30k or even a million k miles on carbon rims, but it is in the end anecdotal as are observations that "others used carbon rims for x miles problem-free."
edit: off the top of my head i remember that when you mount up the carbon wheels with all the bearings and bolts etc they actually weigh a touch more than the s marchinesis....
Sometimes the difference isn't in the total weight, but the weight distribution as well. I am still looking for the actual weights of the BST without rotors/bolts, and OEM S to compare, and get true weights. For the R1, I dropped 27 pounds of rotating mass switching to BST/titanium bits/rotors, etc. I know it's MUCH closer for the 1299 S wheels, but I am looking for actual weights.
With that said, weight distribution is key as well. The weight of BST wheels are in the hub, not the rim vs. weight of the S wheels being most in the rim. I have already changed my rotors/bolts, and lost about 4 pounds from the front, and move more weight to the center.
Simple physics in terms of rotational mass and weight distribution. Look at a centripetal clutch or ice skater, the tighter the mass, the less effort to spin. The more weight in the center, the easier to offset (turn) the bike.
Now, price is part of the process... Talking about cost/ounce. But I have read that BST wheels are about 4 total pounds of savings from OEM S wheels. But weight distribution would also be a big part.
It really comes down to what you are willing to pay/ounce as to whether it's worth it.
I wouldn't hesitate to run a carbon wheel that is DOT certified like BST. Also the fact I have hit my fair share of potholes, debris, curbing on the track, and I have zero issues with the wheel.
Also I have seen a BST rep bounce the wheel on concrete in person. Can a carbon wheel shatter, yes. Can an aluminum, magnesium, yes. Just depends on the amount of force applied. But in daily riding, and track riding, I personally never had a single issue.
Then jump over to bicycles. I have been running carbon wheels on a bicycle for years, and never had an issue as well, even with my overweight self losing weight.