I've never heard that warning in the 2 years I've been doing yoga, either in a class or a video. Of course for me, it's a moot point!
Even without any medical training, the "fluids flowing backwards" sounds kind of silly to me. Perhaps the energy thing is true, but that wouldn't stop me either.
But, I can't do a headstand or handstand. I do the shoulder stand and leave it at that.