I've never had any problems at all with thread from wooden spools, including some that I know to be over 60 years old. I think the parts most apt to be weakened are the outside that has been exposed to light and the inside that has been in contact to the wood.
I do not use mine for areas that will receive a lot of stress, like jeans repair, but, since my functional quilts are all longarmed, I use it for any and all parts of my quilt tops and wall quilts. I have some beautiful silk thread from post-war Europe that I sometimes use as embelishment. The colors are to die for!