I cut a bunch of red elm last July. It is a great bow wood but I did break three of them at first, all tension breaks. The next few I've done I chased a ring down and/or backed them with raw hide and they are the fastest, lightest bows I've made. On all of the staves of RE I have the sapwood seems to have really thin brittle rings down about three rings or so, deeper than that and they are a bit thicker and seem much more elastic. Of course this only pertains to the staves I have any others would be different.
Elm is usually quite forgiving. It has somewhat interlocking grain like hickory.
Mark