Wood is clearly not the best material for an archer’s thumb ring. It might splinter and it might break with little or no warning. The video below shows a hammer test on a cumaru thumb ring. I’ve come to trust some ironwoods a bit less for the purpose of thumb rings, but this little guy was super tough and took quite a beating before breakage. Since I started crafting thumb rings, I’ve noticed the same species of wood can vary greatly from one piece to another. One good thing about cumaru so far, is that it seems to crack or break slowly. Whether that’s the case at all or if depends on moisture content, I’m not sure. For modest use and especially for lower poundages, eg <45lbs, they seem to do well. For consistent use over 45lbs, I suspect the risk of breakage is formidable, but this largely depends on the individual piece and species.
Update: What I’ve learned since posting this many months ago is that the darker the cumaru, typically the more brittle. It is the more pale pieces that seem near indestructible. And it is such that I have chosen to make my ILF ambidextrous riser from.