The problem of unpredictability of a flight control system is characterized by handling quality cliffs and pilot induced oscillation (PIO).
Although PIO is a known problem of non-fly-by-wire flight control systems, time delays, handling qualities cliffs, unpredictable flight characteristics, saturated control systems, the attendant PIOs, and loss of control are characteristics of fly-by-wire flight control systems.
The difficulty in the C-17 program wasn't simply that a handling qualities problem existed in the roll axis.
The purpose of the experiment was to gather data on aircraft handling qualities models (good, bad, and ugly) to correlate in-flight variable simulation data from the original HAVE LIMITS and HAVE PIO open air programs with simulation data.
Experienced pilots unconsciously feel the natural/predictable modes of an aircraft and successfully compensate for poor handling qualities.
Without training or comparisons, it may be impossible for pilots to gauge the degree of compensation, especially with long-term programs and programs where handling qualities have improved gradually over time.
Test pilots need to constantly recalibrate their awareness of aircraft handling quality differences and compensation.