The main thing with a thumb ring is a completely different way of aiming. As mentioned before the impact point is way different if you just aim the same as when using a finger release.
once again,at the expense of sounding argumentative,i disagree.I think the mistake that most make is,they assume,that when the thumbring is used,the asiatic style is used as well.
I truly believe,based on personal research,that a thumb style hold and release was more widely used than previously believed,by American native people.I believe it was used in a more shorter draw style that had the same aiming,or lack of aiming style,much like many use today.I personally,can be fairly accurate,with this style,out to 40 yards,so far.I played with it here n there over the years,but recently started giving it serious consideration.