Tucumseh said:
the physical safety of each aircraft system may be safe in isolation, but functionally unsafe when mated and used.
Which is presumably why the QinetiQ report said:
The fuel system for the Nimrod is understood as is the fuel system for the Tristar. When these systems are combined, however, during AAR, the mated system is in effect a new third system that it is believed, will behave differently from the separate Nimrod and Tristar fuel systems. While some activities have been undertaken to understand how this system responds, further work is required to understand how the mated system responds and behaves during AAR operation.
The AAR system had been in use for 25 years when QinetiQ made this statement.