They were rock solid when it was new and for many years, was only at the very end of its life at Chessington they started to move and nothing like they do now.
Of course it is safe but I do find it unnerving, there are a hell of a lot of forces on those restraints and it's always in the back of my mind that I'm likely to be the heaviest thing they've had to deal with for a while when I ride, if one of them is ready to give up my bulk might just tip it over the edge!