If the head worn display is meant to perform tasks that today's HUD's do, a highly accurate and low latency head tracking system is needed, as someone else mentioned, to keep the position and motion of spatially-referenced items properly scaled and aligned to the outside view. Not trivial.
Also not trivial is the design of display modes to cope with on-axis and off-axis viewing angles, the dynamics of the same, while not contributing to pilot confusion and/or distraction.
The possibilities are very attractive, once the human factors are properly sorted. For every cool thing you can do, there is typically a good reason not to, or at least not to do without very careful testing.