Early Sikorsky FBW development
Good question Sycamore. There were two of us that did most of the front seat flying. The learning curve for flying the 4 axis system was very flat. By the end of each flight we were pretty accomplished flying basic tasks. Little of that was retained between flights. More complex tasks, ie. descending decelerating climbs and turns, quick stops, precision and confined area landings were more than difficult. To say the least we were operating in the lower right hand corner of the Cooper Harper Scale!
The 3 plus one configuration was very similar to a basic helicopter control system. Similar to flying in the front seat of an AH-1 Cobra. Having yaw control on the side arm controller resulted in unwanted yaw control input while performing more complex maneuvers. Having the ability to make small trim changes in collective from the cyclic was a plus.
I spoke with Bill Dana of X-15 and F-16 AFTI fame. He believed that most flying tasks could be achieved by ensuring that the aircrafts basic handling qualities were optimized.
Hope this answers some of your questions.