CSS3 2d/3d transforms have a serious side effect. They cause the object to act as a containing block for fixed positioned descendants. This basically means that you can't use fixed positioned items as a child of such an element.

For instance, look at navbar-fixed-top. You might want to show .navbar-nav outside of a .navbar, like is done in Jasny Bootstrap's .navmenu-offcanvas, which doesn't work with the transform (see jasny/bootstrap#278).

If a user want 3d acceleration to smooth out some transition effect, they should add translate3d themselves.