Thanks Harry. It seems that, on the flat, the rider is often exhilirated by the power boost and actually works even harder than indicated here. For example, my wife often settles down in ECO mode to about 27 km/h on the flat, which indicates she's putting out about 170W.
On the hills, though, the data suggests that she's not happy going above about 190 W, no matter what power mode she's in.
In either event, as you say, the rider is definitely working hard.