Qi Men Dun Jia is older than Bazi.. it is older than Feng Shui.. it is that old, but up until today, it is still relevant albeit we're no longer at war.
I would surmise Qi Men as a tool to forecast the potential outcome of planned actions. Used correctly, its accuracy is astonishing. I frown upon peers who treat Qi Men as an oracle. Questions like "what if I did this, will I get that", when little or nothing has been done. Qi Men is not used to verify inactions or mere thoughts hanging in mid air. Rather, work on an objective, plan and prepare meticulously and when the time is near for the great execution, Qi Men will be there to shed light on the probable outcome. With enough experience, the why, how and when could also be revealed.