A friend and I sinew backed a hard-maple flatbow one damp winter. Come spring, no matter how hard we tried, we could not bring it into tiller. Finally the bow broke. An autopsy revealled the sinew had rotted the surface of the maple in a couple of places , so if you do sinew back it, make sure it can dry out reasonably quickly.
Sinew backing will also make the bow less liable to chrysal, but if you are intent investing the time necessary to sinew back a bow, I would agree with the others and get a decent stave
James