The hidden door on the Kuka arm in Gringotts has had issues, but it HAS worked since opening.
Also, us having a Kuka door in F&F is totally a rumor and in my opinion unnecessary. If F&F will be truly 360 and the exit will have a projection on it as well, they can simply use a similar system to the screen doors they had in T23D. More practical.
Not EVERYTHING has to have the Kuka technology integrated.
With that said, in my dreams I'd love to see a physical helicopter on an inverted kuka arm go around the trams, ready to "attack".