As someone pointed out already DC's failed while hitting the ground after being launched into the air by contact with Massa. For that matter so did Timo's. He got launched by a little bump off track and the subsequent landing destroyed the car.
DC's car fell apart in practice after going off the track in a flat out section, and then hitting the back side of the curbs while coming back across the track, pretty much at full speed.
Now I'm not saying the RB4 isn't fragile, but certainly there are fairly logical reasons for why the failures have happened and they don't immediately point to an unsafe car.
