Firstly when landing and taking off a helicopter it can stick to the floor or have some resistance so it feels clunky and unrealistic, this can make rooftop landings very difficult since when one wheel is on a building the helicopter thinks it has landed and tries to "stick".
Secondly when flying a helicopter with full collective on and you pull a very sharp turn which gains altitude and turns 180 degrees the helicopter accelerates which doesn't appear to be the correct physics.