i have given you the downside of hanging.....now think about it....quick and painless all the focus on the dude or dudette being executed or perhaps a method that allows harvesting...and i hate to say this but if we want to harvest......all you have to do is put them under and harvest....oddly they will die without those organs....is that inhumane?
I'm less about pain during execution (don't care if it happens though) and more about general indignity leading up to it. hanging to me is the least honorable method of being killed, with being allowed suppuku first, then firing squad, the guillontine/beheading, then single shooter, then chair/gas chamber, then injection, and finally hanging.
The problem with death from harvesting is you are asking a doctor to kill a "patient"