This is actually a common bug with webkit-backface-visibility:hidden and although it fixes and stabilizes animations because it triggers hardware acceleration it can have detrimental effects.
There are repainting issues on positioned elements and in some cases position:fixed elements that are children will lose their fixed positioning.
Some of this seems to be by design because when you animate an element the meaning of fixed children does not really compute when you may be rotating or transforming the element.
Therefore I would not advocate adding -webkit-backface-visibility to the body by default unless it was to fix an animation bug in the current project.