Carbon's weakness is damage.....you can scratch them up, but if you get any strong impacts or deep gouges, the frame is basically toast. (You can have some cracks/damage repaired, but it's very expensive and not usually worth doing it.)
Several years ago, Trek did a warranty replacement for me on an aluminum Fuel 95 frame that cracked. Since they didn't have that frame anymore and refused to downgrade me to a cheaper frame, or upgrade me to the newer Fuel EX.....and this is odd.....they gave me a beautiful Top Fuel carbon fiber frame with a special finish. They didn't ask me or run it through the shop first....they just sent it.
It was a beauty, and nimble and lightweight.....but on the third ride I laid the bike over in a rocky turn and put a big gouge with running cracks in the top tube. Trash. And, of course, with carbon fiber there basically is no warranty for damage.
It actually takes quite a bit to fatally damage a carbon frame, but when you do, you'll find yourself shopping for a new frame. I think aluminum is the way to go for most mountain bikes.