Vishwakiran Nambi does a comparative study between the yogic asana and headstand.
While both the techniques look similar visually, anatomically the weight and alignment is different.
The Sheershaasana provides a cushion for the head with the palms, the deltoids and bicep takes majority of the weight. The head hip and feet are in one line.
Headstand uses a tripod like alignment, with the elbows and wrists in one line making a 90 degree angle with the shoulders. If done incorrectly, weight may go directly to the skull, the correct technique is only to keep the head for support while the deltoids and elbows do the work.