Rustle
Well put, I nearly added that the problems with lack of familiarity really start when something goes wrong or when you are flying the aircraft near the limits of its normal envelope.
However it is interesting where the line is drawn. Consideration is currently being given to whether or not an aircraft with a glass display should require a type rating - there are many instructors who consider it should. So for example a pilot accustom to flying a conventionally equipped DA40 would not be able to fly a DA40 with G1000 - same aircraft of course, vastly different systems though. In the same way train on a DA42, and you cant legally fly any other twin.