To me, the biggest problem would be how to use any information Airbus gives on the 350. Is anyone, ever, going to believe a new Airbus type will be delivered on time and with performance that matches the launch retoric?
And the 787 and 747-8 use the same engine, do they not?
The jury also seems to be out as to wether the 787 will be a common type rating with the 777, so cross crewing potential comes into it.