I don't agree, flight manuals do not list/discuss idiosyncrasies of aircraft, there could be a gap between info in the flight manuals and the practical skills needed to handle the aircraft.
This point of view interests me, as I see it differently. It is the stated objective of the certification process of aircraft, and the drafting of the flight manual, to specifically prevent this situation. Anything which is unusual about an aircraft (which is certifiable) is supposed to be explicitly described in the flight manual. The aircraft and the flight manual should be "figure out able" for a pilot qualified on that class. Sure, the new pilot may not fly it like an ace right off, but simply safely should not be in question.
Primary systems are required to be straight forward, right down to the position of instruments and controls, and their motion and effect. Very old aircraft could differ, though really not a lot, for my experience, and secondary or emergency systems can vary quite a lot, so reading the flight manual is pretty important for those. But flaps, landing gear and C/S prop, in their primary function are totally straight forward, If you do nothing it will still fly safely (other than landing with the wheels up). Do you need to train a pilot to know to extend the wheels for a land landing? If you do, the pilot's reading the flight manual and checklist will be that training.
The six tank fuel system of the Cessna 310 comes to mind. It's not common - indeed rather odd. But, it, and it's operation are very well described in the flight manual. You can be trained to use it, or you can just read the flight manual, and follow the instructions.
Of course, the forgoing is predicated on a well trained pilot. If a landplane pilot is trying out an amphibian, they should be getting training for that different class of aircraft. Not that the flight manual for the amphibian is necessarily lacking in describing its operation, but the operating environment is different, and that is beyond the scope of a flight manual in some cases. The same would be the case for the 172 pilot taking it into IMC conditions - the plane is the same, but the environment sure isn't, and special training
is required for that operation/environment.
Speaking of idiosyncrasies, the Cessna 303 was AD'd prohibited in icing conditions - even though certified for known icing, and equipped. This is definitely "Need to Know" stuff (ask me how I know!). An idiosyncrasies of the highest importance, but well explained in the flight manual supplement for the aircraft, and presented in a placard - training not required.
Without being argumentative, I'm interested to hear examples of idiosyncrasies or gaps between the flight manuals and the practical skills needed to handle the aircraft, for a given class of aircraft - perhaps there is something I should be learning.....