Why did the Boeing 787 switch back to separate IRS systems?
There were also other practical end use issues as well. Having worked in the manufacturing and repair of theses fault tolerant units, one of the benefits was supposed to be the ability to plan maintenance. You could have a gyro or acceleromter voted out and still be able to take off and fly, since the F-T ADIRU had six gyros/accels and the SAARU four gyros/accels. Problem was the airlines weren't really taking advantage of this. They'd wait until you'd have two gyros or accelerometers voted out before and then be unable to take off due to restrictions. Some of the sensors sat voted out for years, with no replacement occurring.
Of course the airlines pointed the blame back the design. The F-T ADIRU was the third most expensive part of the 777 behind each engine. Most of them also did not carry spares for their repair inventory due to the costs of spares. Spare F-T ADIRU a couple years ago were running north of 2 million a piece. They found that the aircraft had less down time with a split system then using the fault tolerant system. Largely because they had ample spares for the split systems due the spare parts costing 1/10th the cost of the fault tolerants.