The Cyclists Touring Club did a survey of bar breakages (mostly Al, and not all ultra-light). The conclusion was that bars break mainly in France. Apart from that, there seem to be no common factors.
I wonder, with carbon, if the clamp systems need to be adapted. Principia have specially prepared seat-tube to accept a carbon stem. The slot for tightening is spiral. Maybe stem clamps need to be feathered to distribute the stress.
The use of shims seem to have gone out of fashion with all bars, yet this light, simple system will prevent scoring on the tube.