The horizontal hold is actually an advantage when it comes to dealing with the weight. The problem with the horizontal hold is its postural asymmetry and its reach challenges that cause ever increasing problems for shoulders, neck and hands as we age and our flexibility decreases. Not everybody encounters these problems, of course, but many do.
There are many vertically blown embouchure flutes around the world, so there certainly isn’t a problem with the acoustic aspects of blowing against the returning wave. I personally play quite a few different vertically held embouchure flutes, including quena, quenacho, various kinds of kavals and neys, rim-blown Anasazi flutes, shakuhachi and dongxiao, among others. Not well, of course, but well enough to know that acoustically it works very well … with the right embouchure approach!
It is not difficult to take one of the above flute styles and change the tone hole lattice to play a diatonic major scale, like a keyless Irish flute. But when you do this, the problems you first run into are ergonomic. Specifically, you need to adapt your “grip” so that the flute doesn’t slip through your fingers and fall to the ground when you relax. You don’t get the benefit of opposing pressures at lip and various hand contact points that we utilized with horizontally held flutes. Instead, you need to maintain certain patterns of finger contact which are not immediately compatible with the finger patterns you use on a horizontally held flute, especially when playing ITM at speed.
In other words, you need to retrain your finger work, and it is not clear exactly how to do this most effectively. The other world flutes overcame this problem, of course, often thousands of years ago, but they did so with a different scale and tone hole lattice. I think we could do the same, relatively easily, but it would mean redesigning the fingering system (like piper’s do, say). The holy grail would be to be able to maintain the same fingering style so that people could easily switch back and forth between horizontally and vertically held flutes and whistles.
Of course, we already face this problem to a certain extent when playing low whistles. But it is surprising how much additional stability and control you get in practice from a whistle mouthpiece held between the lips, compared to a flute embouchure resting lightly against the lip and sensitive to the slightest movements.
Also, the weight of a vertically held flute becomes a critical factor. The heavier it is, the more you need to grip it to prevent it falling, and the more this interferes with dexterity. Making vertically held flutes lighter is a huge benefit, much more so than with horizontally held flutes.
