Bike porn

Like all bikes, they vary. Some are badly compromised in the areas of utility and durability, many use components with rider weight restrictions, but some pretty light bikes are made only from components proven on the Pro tour, even in the Spring Classics, e.g. Cervelo R3SL is Roubaix proof even at ~800g, Lightweight Standard wheels have been ridden on many TdF mountain stages and only seem to break if you hit a dog.

I can't recall seeing anything under about 5.7kg that I'd ride in anger, but I'm a 90kg sprinter, not a 60kg mountain climber. Oh, and you're not 'flying' down a mountain if you're only doing 40mph, you can go faster than that coasting down some of the short descents in the Chilterns and Cotswolds.

Cheers for detailed response!
TBH if something can stand up to being sprinted hard on the Roubaix then that pretty much answers my question.
I guess they are all fit for the purpose of riding hard (but in a smooth manor) and can take general road use, but any serious pothole/kerb/car/concrete interaction is going to result in failure.
I do think of carbon as very fragile, but then, I'm riding a 40yr old steel lugged frame and have to admit, every time I ride/wash it, I do inspect every lug & joint for signs of cracking or fatique (and cranks, snapped a crank before and i'm lucky to still have a leg to pedal with!), just incase :p