"If you a sitting in the cockpit Boeing or Airbus and you do not know what the aircraft is doing, then you should not be sitting there"
Generally speaking true, but as aircraft have become more and more technologically advanced and computer-dependant, coupled with less training and increasingly sparse information in the manuals/FCOMs compared to older models, it is not unusual to hear even experienced training captains utter the words, "I've never seen it do
that before".
Anyone who has flown recent generations of transport aircraft for any amount of time has seen them do things that they are not supposed to do and are not in the manuals, and that when described to engineers/Chief Pilots are met with a shrug. What is important to me is not necessarily knowing
why "it did something" but rather what it
should be doing, and be able to rapidly intervene and put things right if necessary, by reversion to a simpler mode of automation or manual flight.
That is what determines whether you should be sitting on the flight deck in the first place.
I do believe however after seven years of flying A and seven years of B, that company B's products are generally designed to be more intuitive and pilot-friendly. IMHO of course.