Acute little pose in itself. I teach it when I find someone still struggling with the crow, or for children. However, even I tend to hold it only for a few seconds, since just getting in and out of the pose seems like so much fun. But it is when you hold a pose like that for long then you appreciate that it may call for a lot more in terms of strength -- from the hands, shoulders, and of course the hips. The interesting bit about all arm balancers is that they call for hip strength -- the hips press the legs into the arms to hold you in place. Though advanced students start off their arm balancers naturally (and often unconsciously) with this pressure, for beginners this kicks in only later -- when they slip and realise that they need to lock their legs into the arms to stay afloat longer. That is when they realise that a powerful and well held arm balance needs a knee lock (initiated by the hips). It is when you hold arm balancers for long that you enter yoga zone, even in a simple, sweet pose like this one:)
Of course, the tougher bit is to move from the headstand or scorpion into the duck and back... have not tried it yet, but feels like it is time I did that!!