Carbon Fibre Mountain Bikes
Personally, no, I have not had any problems w/ carbon fibre mountain bikes. I am currently on my 5th such steed (Trek Fuel EX 9.8) and it is going quite well. My thumb ... not so well. But, that's besides the point. Of note ... I weigh 145 lbs and ride the bike for what it is meant. 
I have seen many broken carbon frames. Like wise I have seen many broken aluminium frames as well. Typically when I see a broken frame it is caused by many of the same things. 

a)Riding a bike outside of its comfort range. If you have a 26" carbon fibre hardtail, do not think it would work for jumps or all mountain riding. You will break it. Don't take your 4" travel carbon race bike to the downhill course either. Use common sense. 

b)Maintain your bike. Use a torque wrench and carbon prep. Check linkages. Keep it clean. Those who don't tend to see things (or miss things ...) break with greater frequency. 

Scratch depth has alot to do with this. A simple scratch ... no, it doesn't render it useless. I have many scratches on many carbon bikes. I have paint chips etc... and they are still a-okay. What I would be aware of is a 'scratch' on the handlebar that could result in stress risers and eventual breakage. Again, this goes back to my points above concerning maintenance. Regular maintenance (of any bike) is key. 
There are also advances in carbon that companies are indicating have created a much stronger frame. The following are two I am familiar with but I am SURE there are more. 

http://origin.trekbikes.com/us/en/story/fuel_ex/technology

Cannondale has 'Ballistec Hi-Mod' carbon which I believe is supposed to be similar.

OCLV is just another way of 'laying up' carbon. I have had many OCLV bikes and have enjoyed them immensely. I am sure I would have enjoyed a monocoque frame or a mitred frame as well. In my mind it is the companies warranty that matters most. Will they stand behind you if the frame fails from reasonable riding? In my experience, yes, Trek will. But, I'm not here to push Trek. Buying a well known frame, with a strong warranty from a shop you know will go to bat for you is what I would recommend. 
In regards to carbon lay up, yes, it does matter. The ability to strategically lay the carbon (or in terms of the new BMC Impec weave the carbon) allows engineers to create strength where strength is needed and to remove weight were less material is necessary. It can be likened to the 'butting' of steel, aluminium and titanium frames. 

Sorry for the Trek angle throughout this but as the question regarded OCLV and I have experience with both, it was inevitable.