The drawback is myriad knock on effects for seemingly 'unrelated' system due to a fail/fault in one system, that never existed in second gen airliners.
Don't have the MEL infront of me at the moment, but from memory if ADIRU 2 is U/S, take off cannot be performed in Config 1+F. How on earth can failure of such a seemingly unrelated system have that effect ?
Answer - If erronious airspeed indications were recieved there is nothing for the flap overspeed protection to compare with, and the flaps could retract at the wrong time. One incidence of this happening I think.
What about those spurious ECAM warnings that occasionally come up and disappear all by themselves, also the STS message which pops up briefly but you are never quick enough to push the button to view it before it vanishes.
Welcome to third generation airliners.