I was chatting with an FAA Inspector about 6 years ago, re. the 787. He said nobody at the FAA wanted their name on the certificate! He said they are very worried about what will happen to the pax, if/when that plastic jet catches fire, ie. toxic fumes, to both the pax and firefighters. Also, he said they are concerned about what will happen if/when one crashes.

An aluminum tube will crush and absorb a lot of the impact, and many people will surive the impact, but a carbon tube won't flex and bend like that. It will either bounce up, hard, and snap everyone's neck, or explode into a shower of sharp splinters, neither one of which scenarios is going to be good for live people.

Now...throw in some exploding lithium batteries...