The situation was specific to the B777
, and was quite unique, one ADIRU was feeding the stand-by so if that ADIRU unit was inop they no standby
A Tad adrift here sir, the SAARU sends data the the Stby Attitude Indicator, not the ADIRU. Your point however is valid, although I can not recall any in flight SAARU failures, but system integrity is certainly increased with the ISIS